Tyranids - Early Reaction from a Long-Time Nid Player
So I played 40K briefly when I was in Junior High and early HS. This was second edition. I played Tyranids. When I picked 40K back up again in my mid-20s, I initially ebayed a Necron army b/c it was cheap and I was told they were easy to learn, this was in 4th Edition. Within a month I had a Tyranid army again, and I've played them consistently ever since.

I'm disappointed in what we know so far about the new codex (which is almost everything). I don't think the sky is falling - I'm sure a few good lists will come out of it - but it's roundly bad at dealing with the 2 "best" codices of the moment (Eldar/Tau), and perhaps also even worse in terms of internal balance than it was before.

Here's my initial thoughts on it, keeping in mind all of this will be revised / may change based upon holding and playing with the total codex (don't lose your minds yet folks) ...

Overall:
The first thing you'll note is Tyranids no longer have access to book powers. This combines with no access to allies to make them unique among 40k armies - they have 0 book power access and 0 ally access. People keep saying supplements may change this, and they may. Let's look at the present and be happy if the future changes things, however. Right now, Nids are the only codex that does not permit you to access the rest of the game via allies, and does not permit you psychic flexibility in terms of tree selection and varied powers.

Instead, they have access to the Powers of the Hive Mind. OK, cool, well these are probably awesome since there are no other powers available to them besides these.

Psy Powers
Primaris - +6" Synapse range. This is a really boring primaris, but will probably be important, as Tyranid synapse generally got more difficult to field, and one of only 2 "hide-able" synapse Independent Characters were deleted from the codex altogether. Meanwhile, the other one (Tyranid Primes) received a greater than 50% base points increase with no changes to its functionality or power level. Basically, your functional synapse is going to feel overpriced and/or suboptimal with a few exceptions, so having as much extension of synapse as possible may actually be valuable.

1 - Catalyst - Blessing. Psyker's unit and one other friendly unit w/in 12" gains FNP. This is actually pretty decent.
2 - The Horror - Forces a pinning test with a LD modifier, 24" Malediction. This is the Broodlord's base power; we'll talk later about why it's pretty bad on a Broodlord against what people are kneejerking to. IT's useful enough situationally.
3 - Onslaught - Same basically; 24" blessing that lets a unit run+shoot. Situationally useful.
4 - Paroxysm - 24" malediction that is -D3 WS/BS. Randomly acquired and a straight nerf of its older version.
5 - Psychic Scream - Doom of Malantai Power basically. 2d6+2 - LD wounds with no armor/cover, nova 6" ... this is a fine power if you can deliver it, but there are no more spore pods, which means accurate delivery requires combos such as lictors, and ... well, you might use it you might not in a given game, which is what this boils down to.
6 - Warp Blast - While ML2, that's not too inhibiting given the way it is applied in the codex, but it's largely unchanged (I think the AP of the Lance went from 1 to 2, though, so that's disappointing).

Psychic powers are basically "OK," none of them are all that bad, the Primaris is a little underwhelming.

Army Rules
Shadow in the Warp is -3LD to enemy psykers w/in 12" more or less ... this is a little better [for Tyranids] for sub-LD10 psykers and a little worse [for Tyranids] against the primary type of psykers, LD10. Not a huge change as I'm given to understand from those who are more mathematically committed than I am.

Synapse - Fearless/auto-regroup, nothing changed. Eternal was not added, which is a little disappointing from the perspective of making the mid-range T4 bio-constructs useful in take all comers gaming.

Bioweapons
Largely unchanged, honestly. Scything Talons were nerfed (no longer offer any form of re-roll to hit, and are just "weapons" with AP6). The AP6 is nice I guess, if you want to kill 16% more orks and kroot and tyranid. But you didn't struggle killing those as it was. Rending Claws were buffed, basically staying the same but gaining AP5 to boot.

Bio-Upgrades
The various common upgrades got a little better in some cases (Adrenal Glands = furious/fleet instead of just furious) but generally also got more expensive. The shunting of points from the creatures to their often-mandatory upgrades accounts for a lot of why the dex doesn't actually play cheaper / with more models at a first glance. Some upgrades were stripped from the dex entirely, like the armored shell ... you can't get a real 2+ save unless you're a Tyrannofex.

Bio-Artefacts
The "fake" 2+ save is here ... an expensive ugprade that lets you have a 2+ for just an assault phase, every other assault phase. Hooray.
The bio-artefacts as a whole are generally awful.

Maw-Claws of Thyrax - for 10 points you get rending claws that give you Preferred Enemy during the game against things you've killed one of with that model, for that model. So you get to re-roll a few 1's. Don't bother.

The Miasma Cannon - for 25 points you get a weapon that's not really any better or worse than most of the other already-available bio-weapons. Either a 2+ to wound AP4 template or a 2+ to wound AP4 blast. Skip, far too expensive (or you could argue, far too weak and non-differentiated from standard bioweaponry).

The Norn Crown - +6" synapse range (non-psy-casted Dominion, or a boost to Dominion) is alright ... UNTIL YOU REALIZE IT COSTS 40 POINTS. Horrible.

The Ymgarl Factor - +1S, +1A, or +1 save every assault phase, for just the assault phase, and you can't repeat the same one twice ... and it costs 40 points. AWFUL.

The Reaper of Obliterax - This doesn't make a lot of sense to me either. It's a Bonesword with +1S and +3I. The problem is it costs 30 more points than a Bonesword, clocking in at 45 total. Again, skip.

I don't see any of the bio-artefacts as being worth taking.

Onto the units
HQ
Hive Tyrant - Points decrease to 165, upped to ML2 psyker, but lost access to anything but Powers of the Hive Mind. The double devourer flyrant you'll see poeple harping on is still fairly bad, a unit that was used a lot in 5th Edition but never won major events and never concerned serious players. It still clocks in at 230 points at least to field this build-out, so it's more affordable but still not at all cheap.

Swarmlord - Mega nerf. His weapons lost their ability to force re-rolled successful invulnerable saves. He lost access to powers like Biomancy which were critical for his points value. He lost one roll on the psy tree (in exchange for getting ML3 instead of ML2, but this is a substantial nerf regardless overall). Fortunately they reduced his cost to 285. WAIT WHAT? They added 5 points to his cost. Oh.

Tyrant Guard - These are about what they were, and are actually fine. Crushing claws are OK on them too, though at a substantial cost.

Old One Eye - Cheaper but largely the same. Doesn't have any special kind of regen. He's still pretty bad. I don't know how to tell people that a T6 W4 3+ save MC is never going to last long enough to apply cool regen or FNP powers. He's really not 100 points better than a stock Carnifex.

Tervigon - OH THE PAIN.
No one should be surprised that "the" Monster choice from the last codex got targeted, but let's see how much.
He gained +1I
He lost access to more than 1 psy power, and is ML1, and lost access to all book powers.
He lost the ability to up his attacks profile with crushing claws.
He lost the buffs given to nearby gaunts, inclusive of Leadership, Poison, Furious when he has those.
He retained giving Counter-attack to nearby gaunts, but of course they aren't using his Ld anymore.
When he spawns gaunts, he can still burn out and he spawns the same quantity, but the gaunts themselves cannot move or assault upon arrival.
He can only be made a Troop for each 30-model Termagant squad you take. Though termagants became cheaper, that's a 70 point uptick to enable Tervitroops right off the bat. That's ok, though, b/c he probably went down in cost due to all the dramatic nerfs right?

Wrong. The "Cost" for that dramatic ... improvement??? 35 points. Tervigons are now 195 base.

Tyranid Prime - More pain.
Basically exactly what it was before, but possibly a lost wound (I can't recall off top of head if they were T5/W4 or T5/W3). Regardless, let's assume he's the same. The cost for no changes? Increase his points base by over 50% from 80 to 125. Really?

What's important is the Prime is the codex's only IC synapse, which becomes even more important due to Instinctive Behavior being generally a much worse thing to have happen.

Parasite of Mortrex - The other IC synapse in the codex has been removed from the game.

Deathleaper! He's HQ now! Cool! Wait he's more or less the same. Snap shots only to shoot at him, which is an upgrade (And also means he can't be flamed or blasted, which is kinda cool). He costs a little less I think also, and is pretty decent still stats wise. Not an IC, so no hope of him being anything other than a stationary target. Still applies a LD debuff to enemy IC of choice. Overall actually pretty decent. Hooray!

HQ Summary:
Tyrants are still IMO a subpar unit that costs too much and is too targetable, requiring massive points investment of guard to make them even last a while unless you're playing against pretty subpar or niche opponents. They are cheaper, so at least there's that.
Tervigons are utterly terrible, overcosted and with almost no serious contribution to make to the game. The Swarmlord is not worth his points, at all. Old One Eye is terrible. You may find yourself often taking points for a Norn Crown Tyranid Prime despite the absurd overcosting ... and Death Leaper maybe ... b/c what else are you going to do? Besides, you'll need to hide your Synapse in 30-model gaunt units; none of the other HQ or synapse units can survive sustained fire from ElTau, especially not at their costs.

I don't think a lot of peopled owned a Tyranid Prime or Death Leaper if they were 5th Edition players, so you know, buy one of each. Otherwise you probably already own anything you'll use or try to force using from this (Tervigons, Flyrants). Cool, almost no cost!

Troops
Tyranid Warriors - As far as I can tell, these more or less didn't change. You can buy 4 point frag grenades for them. They're horrible. There's no way to give them eternal, or to increase their T, so they're super fragile to anything that can double them out (like, I Don't know, cover save ignoring re-rolling to hit S8 pie plating riptides w/ trick commanders? OH THOSE AREN'T COMMON). Nobody took Tyranid Warriors in 5th, and nobody will take them in 6th, outside casual gaming in environments where the social contract tailors things to allow their functionality (which is fine I suppose, but why design units / games where things don't work unless you have long conversations and hit-or-miss gaming sessions to figure out what's ok or not to bring from the base legal codex??).

Genestealers - These basically didn't change. They're still awful. Broodlords get The Horror by default, instead of the Stealers getting some kind of Flesh Hook access. On face value, you'd think "Well cool they can pin a unit before assaulting it, so it won't benefit from being assaulted through cover, it's kinda like grenades!" The problem is, The Horror is a Malediction, which means the Broodlord needs to have LOS to the unit you want to maledict a turn in advance. In that interim opponent's turn, the Broodlord must subsequently survive incoming fire (and he only Looks Out Sir on a 4+), and the unit you want to assault must still be in LOS after having access to a full player turn to move out of LOS of the Broodlord or simply block his LOS. So, nice thought, but totally impractical. Let me know when they release an errata that gives Genestealers some way to survive being exposed to enemy fire for a turn.

Termagants - These are basically what they were, only a little more expensive if you want to give them furious/poison. Last edition they were 5 points, 6 for furious OR poison, 7 for both. Now they're 4 base, 6 for furious OR poison, 8 for both. Yes, furious gained fleet and is thus better. Termagants are still your mainstay, but it's trickier now b/c you don't get to apply poison with Tervigons, so you're going to have to either sac fleet/furious or spend 8 points/model to have them be flexibly able to deal with a variety of threats. You'll possibly need to anyway, b/c ... well, that's this codex's problem. Not much in it is any good.
Unit composition did improve somewhat, as you can selectively equip the brood w/ Devourers, thus building ablative non-devourer wound suckers into it. Some things you can play with here that might be OK. This is cool b/c you don't have to buy any models if you're a long-standing Tyranid player.

Hormagaunts - Same old same old. Termagants do exactly what Hormagaunts do, but they do it better, and they have S4 AP5 guns.

Rippers - These got more expensive. I have no idea why. They don't score, they're ID'able, they don't hit for anything. Ummm ... continue to sit on shelf and look pretty!

Troop Summary - Nothing changed, literally. Gaunts are your only good Troops. Have fun, you don't have to spend any money to get set for the new codex!

Elite
Hive Guard - They gained access to a new gun for +5 points that's Haywire. Their old gun basically remained the same. They lost 1 Ballistic Skill. They did not gain skyfire-ability or anything. Cool, cool, sounds like they should drop to 45 points and have a +5 (I guess, not sure why) for the Haywire gun. Wait, no, +5 points base. ???????????????? Yet still for many list types will be mandatory. That's cool, LESS NEED TO BUY NEW MODELS YET AGAIN!

Lictors - 15 points cheaper, 100% still crappy. Some people will find uses for them in gimmick style lists with their deep strike teleport homer nature or whatever, but they're still really really dumb/unfluffy. WE SPENT WEEKS CAREFULLY SECRETING OURSELVES AWAY AND PLANNING FOR HTIS ONE MOMENT!

/leaps from cover ... SURPRISE! Pauses, lets opponents carefully arm and aim their weapons, BS'es with buddy Lictors b/c they're forced to operate in ... SQUADS? What? Anyway then they get shot, but they sure did surprise those guys!

Zoanthropes - These guys were hit hard. Previously they could use warp lance/blast each, with lance being slightly better. They rolled psy tests separately, which meant targets rolled Deny the Witch separately, which meant in general better odds of at least a couple shots getting off and getting to actually roll to hit against their target. Now here's how they work. They still get lance/blast, and they fire it as a unit. A unit of 3 fires 3 shots, 2 fires 2 shots, etc., but it's just one Psy Test (Brotherhood of Psykers). So you have a 24% chance right off the bat of either failing the Ld10 test or the opponent DTW'ing on a 6+ (assuming they aren't psykers of some sort, and Zoeys are only ML1 so any 2+ is dtw'ing on a 4+). Then, you have to roll to hit, so each shot then has a 33% chance of missing. Then you have to actually pen/wound, get through cover, etc. These are terrible. They can roll an additional power on the hive mind chart I believe, but ... ugh. Don't bother. They have 2 warp charges a turn but their main weapon is WC 2 so you'll only use that other power if there's nothing to shoot at with your 6 wounds of marine-level saves and toughness hovering about looking silly.

Venomthropes - Basically the same, but I think a little cheaper, and they got buffed from a 5+ cover save to an aura applying Shrouded. This is actually substantial in the upgrade department, but still doesn't address the fact that Venomthropes are independently targetable and not very durable. Competitive-minded/smart opponents will simply shoot these over-large T4 models with their large LOS footprint, applying things like trick commanders and serpent shields to the task, and that'll be that. AT least they'll have to shoot them first, vice having to apply fire to an army full of 2+ and 3+ cover saves. They're not terrible.

Haruspex - The new close combat variant of the new ground-monster/oval-base kit. He has one more initiative than the Tervigon, 1 more strength and otherwise the same fundamental combat stats. That's right, the combat monster is a slow-moving WS3 S6 guy with 3 attacks and a tail weapon. He's actually quite bad. I don't know what the point of the unit is, I know it has some hinky things that work ok fighting a large blob or squad, but they aren't actually THAT great at said task, and all of it is hampered by the fact that this big ole killer monster hits guardsmen on a 4+, and there's almost no opportunity in the entire codex to get re-rolls on things like to-hit rolls. This guy isn't worth it compared to any of the other elites. He's like the monstrous version of the Pyrovore. Well, maybe he's better than a Pyrovore.

SPEAKING OF WHICH
They made Pyrovores cheaper and have another wound and such. They didn't change them otherwise. They're awful forever. Next.

Elite Summary: Hive Guard, maybe Venomthropes. While you're at it, said Hive Guard were nerfed for no real explicable reason and cost more. Hooray. I don't understand why they wouldn't buff the new model, and some of the never-ordered backlog, to make them decent.

Fast Attack
Shrikes - Basically the same, still bad.
Raveners - Basically the same, still pretty bad, maybe somehow better though I don't really have a way to explain why I think that. I've always loved Raveners, but I can't see taking them over Gargoyles still.
The Red Terror - Hit with all of your attacks, with no re-rolls to hit, and you can remove a model you're in base with, if you're in base with it, and if ... aw fug it just realize it's like 85 points and move on, but be glad they added Red Terror back. Only, then be sad they deleted several models too, so apparently they don't believe in win-win for their players ... you get nothing unless you have something taken.

Flying Rippers - lol
Spore Mines - why are awkwardly floaty things that have to be delivered by a heavy support creature having an orgasm considered fast attack? You know what don't answer that, but move along past this entry.

Gargoyles - these are still the shining star of the FA section, though they were arguably nerfed, though they also guarantee blinding what they attack.

Harpy and Crone - The reason these were unpopular in the past was not that there wasn't a model. They were unpopular because they are a T5 4+ save FMC with a giant LOS footprint and no invul save of any kind. Also, the Crone can only fire 2 tentaclids at most a turn, and that's if he doesn't drool on somebody or vector strike something. Or she, or whatever, I don't know, but both of these are poor units. People may take them anyway just hoping they can I guess do something before they're blown out of the sky by a single quad gun round (4 hits and wounds are likely, and a failed grounding test thereafter would do them in).

Fast Attack Summary: Yet again, stick to the models you already owned if you're playing serious Nid lists, Gargoyles.

Heavy Support
Carnifex - To someone who played Nids in 4th edition, when there was less firepower going around and Carnifexes were only 85 points base, this unit still feels really bad. It kind of is. But there are a couple builds that are not awful now due to the lowered cost. Gunfexes hiding in backfield flinging a S6 large blast pinning and a S9 small blast seem functional enough. You could also run them as dakka.

Biovores - Basically what they were before, good enough.

Trygon - Taking away re-rolls to hit at the cost of 10 points cheaper but more expensive upgrades is kind of a messy exchange. It's hard to be a big fan of these guys still; they show up, fire their little electric stuff of, and die. With reserves manipulation difficult for Tyranid, it's hard to conceptualize the right way to go here, but there might be some kind of combo list with a lot of reserves arriving and using Lictors. The problem of course is there are a lot of things in this game that punk MC's shooting and cc, and you can't get around the requisite turn after arrival of just sitting there holding your junk and hoping not to die.

Trygon Prime - Still meh also.

Mawloc - Upgraded and made a little random at the same time. You can't safely go after large target clusters with the Mawloc anymore, b/c you no longer push models out of the way after you blast up from under them. Instead, you repeat your damage roll from the underground arrival. If things are still left after the second roll, the Mawloc Mishaps. People think these are much better b/c they are cheaper, and I agree they are better for the most part, but whereas you used to be able to safely fling them at large clusters of enemy targets and thus discourage that kind of clustering, now you can't safely do that at all, and so you run a risk of mishap'ing in the worst of ways. Now, that said, you only have a 1/6 chance of an actual super "Bad" mishap result, but even being placed right where they want can mean sticking your Mawloc in the open, under the gun, without cover, and ready for a quick and sloppy death. Not sure how I feel at all about this one yet. Might be hopeful?

Exocrine - Range and BS and some other factors make this an unreliable model at the cost. 6 Plasma shots sounds really good until you realize most turns you're going to be missing with half of them. Still, it's not terrible, you just wish it had a way to get a higher S than 7 on the alternate mode.

Tyrannofex - Basically better, but still an underwhelming unit. Hey look a 2+ save!

Heavy Support Summary - Kinda meh, but maybe the best FOC area? Maybe? There's some potential for various lists in using exocrines, maybe tfexes, carnifexes and biovores. None of them are great units, however. You might actually spend like 50 or 100 bucks getting this up to speed, I guess?

So as a whole what we have is a codex that basically does what the last one does, but does it less well. The psychic powers were all nerfed, most of the good units were nerfed, the units they tried to make into the "new hotness" are not actually the new hotness at all.

IF you're an extant Tyranid player, the nice thing is your investment to make the best reasonably decent list is probably going to be pretty light. In terms of using the rules to sell more models to existing customers (their primary sales model), this looks like a substantial failure on Games Workshop's part. Even players who find ways to be decently good with Nids aren't going to really need to buy many new models to do so. So I guess that's kind of a plus.


PS -
In summary, while every other codex has access to:
3-4 different codices per army via the allies matrix in conjunction w/ formations
Very solid book powers
Power units that can be spammed in a pinch to up the power level of the dex

Tyranid have:
No allies of any kind (at least presently)
No book powers, unique among all codices, and access to very average codex powers
Literally no clear-cut power units within the codex (no riptide, missiletide, wave serpent, gravbiker, etc., equivalent of any sort)