Slann are most certainly worth it! In fact, they more often than not work better than an Oldblood (not to imply an OB can never/should never be used, they can be very competitive) and make the Lizardmen army very strong indeed. Slann are very versatile, they can be used in magic-heavy armies primarily but can be used in combat heavy armies, too (not so recommended).
Since you are skeptical about Slann, I will try to sell them to you by going over pros and cons of the Disciples, kit, cost and tactics (this is the forum for that!) of each bit. The following stuff is based on the way I play, I find it to be effective. It's all written based on objective facts from my games. Others are going to have different opinions, probably. Bring on the long-winded-ness!
Even a bare-bones Slann is a force to be reckoned with. W5 means he can get stabbed a few times and not bite it. Ld9 is the highest you are going to get with Lizardmen and makes them even more steadfast with Cold-Blooded. A 4+ Ward
Sv means you have a solid chance of keeping wounds off him.
There are a few things that are fairly mandatory mandatory to take on a Slann. First is to make him a
BSB. Whether he is in a unit of
TG or not, he can (and most likely) will be one of the more sustainable characters you have. Second is the Cupped Hands of the Old Ones. If you are going to sink this many points into a character, you REALLY don't want a miscast to blow him up or any of those other nasty effects for that matter. Third is the first (and free) discipline, this is a rule-made mandatory choice, but it should (almost) always be Focus of Mystery (Full Lore).
Before magic items, I will get into Disciplines. The first one, as I said, is mandatory and "free". In 2000 point games I will take at least two, usually three (same at 2500). Four is more for large games, 3000+. Our Slann is going to get expensive fast and those extra fifty points are better used elsewhere at 2000. Each Discipline brings a different thing to the table for Slanns. Let's look at each:
Focus of Mystery; the "full lore" Discipline. One of the best ones that the Slann has in his arsenal. The ability to have access to all the spells in one Lore of Magic is freaking awesome. It gives an insane amount of versatility when you know what your opponents army is and even more so once you seen what type of units they have. I won't get into the benefits of each Lore, though. I consider this to be one of the best Disciplines, almost mandatory, especially when taking a magically-offensive Slann (the only true way in my mind).
Focused Rumination; the "free die" Discipline. Another fantastic Discipline. This one is a personal favourite. When casting low cost spells it can cut the number of dicce you need in half. This Discipline allows you to stretch your PD like crazy, meaning you are going to get more spells off. This is a great one for magically-offensive Slann, too. I use this one a lot. I wouldn't say it's mandatory but it is for sure one that should always be considered.
Transcendent Healing; just what it sounds like, you get
regeneration for the Slann. This is a really good Disciple in you run a solo Slann (i.e. no
TG.). This on top of the 4+ Ward, T4 and five wounds makes him damn hard to kill. I would say that there Higher State of Consciousness would be better for a solo but I will get to that. Still a solid choice, especially if you are worried about a solo Slann getting hit by magic missiles and the like (which will happen!). Watch out for flaming attack, though!
Harrowing Scrutiny; Slann gets
Terror... I find this to be THE most pointless Discipline, as well as the the only one truly to be ignored out of the eight. There are three ways to get Stegadons, which cause Terror and they are going to do A LOT more than a Slann with it. A unit of
TG with a Slann is ItP, also. With better alternatives, I have NEVER used this. I can't think of too many situations where it would be useful either. Even on a solo Slann, your points are better keeping him alive than trying to scare things off.
Higher State of Consciousness; the "no mundane attacks, please!" Discipline. This is another FANTASTIC Discipline. This is THE Discipline to take for a solo Slann. It means that the enemy is going to have to commit a character with a magical weapon to him or pelt him with magic and neither is a great option. This one sells itself for a solo Slann, I won't even go on!
Becalming Cogitation; the discard 6s Discipline. This is another personal favourite. Being able to deny enemy wizards irresistible force is nasty! Plus, it can mean that their spells are going to fail more often unless they commit more dice. The 24" range gives the Slann a huge reach with this too, meaning that there are going to be few enemy wizards out of reach, save maybe a
MoT-Sorc on a disc (and even so, he isn't going to want to be THAT far out). The only drawback is that it's only one per turn. I have used this on two Slann in a 3000 point army before, causing even more magic woes for the enemy.
Unfathomable Presence;
MR(3). This is a so-so Discipline. It can be really useful when used on a solo with Higher State, shutting down magic cast on him essentially making him untouchable outside of a magic weapon. It is also useful in protecting a fat unit of
TG from heavy magic if you are really worried about that. I will sometimes take this instead of discards 6s. The only problem with this is that it is going to keep the magic off the unit, which means more of it is going to be focused on your other units.
Soul of Stone; the re-roll miscast results Discipline. This is a great one. This can come is real handy when throwing down a lot of dice. Personally, I like using it with the Cupped Hands; re-rolling for a WORSE effect, then shunting it to an enemy wizard (with the Cupped Hands) is pretty fun to do (especially when they explode!). After you have had that fun in a few games, it is good to have for when you get those particularly bad miscasts. This one is a pretty solid choice. It has saved my Slann many a time form a horrible fate!
The Slann have a nice spread of abilities here that give them the ability to fill many roles in the magic phase. Regardless of what you do, the Slann can and (usually) will dominate magic in any game they are in. After all, they are the masters of magic in the Warhammer world!
Now that we have the buffing options down, let's look at some kit options.
First off, making the Slann a
BSB is mandatory. Just do it. This gives him options to banners, which can improve him whether he is in a unit or not. The better options are Plaque of Dominion, which makes all enemy wizards within 18"
stupid. This another great way for the Slann to further dominate the magic phase. Lower
Ld wizards will suffer greatly from this. It makes a decent alternative to discard 6s if you want to take different Discipline. It does cost the same as one, though, meaning you might be better off just getting the Discipline instead! Another banner option is the Sun Standard of Chotec. It's ability to hinder missile fire against the unit is invaluable. A Slann-
TG unit is going to soak fire like CRAZY, keep those extra few hits (and possibly wounds) off really helps. Remember, too, that the
TG can have a banner of their own; which is a great way to get both! I can't suggest any of the other banners, though.
There are a few common magic items which can be very useful. Power Stones can help in a pinch when that one spell really needs to go off; it's a bargain at its points, too. The War Banner is a fairly common option to drop on a Slann. It gives a nice boost to
CR each round which can help sometimes. I use a Power Stone often, it helps a lot. War Banner is not a personal choice but I have seen and done well while using one. One suggestion I can make is to keep Scrolls of him, that's what his Skink attendants are for anyway!
Enchanted Items section has a few nice options. The Divine Plaque of Protection give a 2+ Ward against ALL ranged attacks. This is just too perfect for a solo Slann, tooooo perfectttt! Combining this with the other good options he has when running solo can seriously make him nigh un-killable. The War Drum has uses but marching a
TG unit isn't always advised and Ld10 isn't need to rally as Ld9 on Cold-Blooded is hard to miss! The Curse-Charm of Tepok is yet another way to make miserable the enemy wizard by forcing him to re-roll a miscast. A final option is the Bane Head. This good when used against a lone hero or when using a spell like The Hunter's Spear, where you can pick a character out (as it works like a bolt thrower). Just remember you can only have one...
99.9999% of the time, your Arcane Item will be eaten up by the Cupped Hands. The Cupped Hands shunts one miscast off your toad and on to an enemy wizard YUO choose! In the event that you don't take that for some strange reason or take a second Slann (at 3000 points), there are some decent options. The Cube of Darkness is one of these. Not only is it a Dispel Scroll but it also has a chance to dispel all RiP spells in play AND end the magic phase. Denying an enemy an ENTIRE magic phase can be potentially crippling, especially when fighting undead armies. I find this to be an under-rated and under-used item with HUGE potential for just forty points. I use in 3000 point games extensively and sometimes in 2500. The Diadem should also be avoided unless you are going super-pure defense other wise you are killing your PD.
As you can see, the option just keep getting better! Slann are sick nasty and totally worth their points. He will make back their points in vaporizing enemies left and right. The only true downside is that these fat toads get expensive and FAST. Despite this fact, they are totally worth it; most of the time a decent set of Saurus character will cost just as much and the lowly Priests won't be able to do nearly as much, at least in the magic phase (not counting too much the brutality of EotG as that is a different(-ish) niche in the army and on the battle field.
Now let's talk tactics (what's really important)! The way I see it there are four ways to run a Slann: with
TG and magically offensive; with
TG and magically defensive; solo and magically offensive; and solo and magically defensive. I find that the defensive options are weaker, less fun and over all lame. Slann is
TG are hard, will be a huge point sink but will be in the battle to the end. Solo Slann can be very good and cut out the cost of
TG leaving room for other things. Solo Slann are a bit under-rated in my opinion; I don't see them a lot. I have been laughed at putting one out before, needless to say, apologies were made after turn six.
TG have to be explained a bit before I get too far into this.
TG are a great way to have a solid anvil in the army.
TG become ItP and
Stubborn when with a Slann. These effects make the
TG an awesome unit. There are some options to make them
unbreakable, too (Lord Kroak and Chakax), but are rather expensive. Chakax is a good option for
TG and, like his fluff, is insanely good at keeping the Slann alive. Sadly, he is really far to expensive to be a serious and competitive option at lower points (although I have used him in 3000 point games to great effect). Sixteen is usually enough in lower point games but twenty is better at 2500+. A full command is really needed, the benefits are good and the Reverend Guardian can take twenty-five points of magic items, which can be useful since he will be expected to meet challenges. Using Halberds is not a particularly good idea, shield-
HW combo will give you a 2+
Sv in
CC (which is a better, in my opinion, than the +1S). They can come in handy though. The Slann gets two advantages here that he won't get solo. First, he cannot be directly attack, as he is placed in the second row, until the front rank drops to less than three models (or everything on his sides are killed and then flank charged but at that point you have other issues). Second, when in unit of
TG he is treated as a Large Target for the purposes of casting spells. This means that he can see over everything and cast on whatever his magic can reach, solos don't get that. It also allows his to cast while the unit is in
CC, another good aspect for obvious reasons. You will want a hammer to support. EotG make good options in 2500+ lists, while CoC, Kroxigor or a small Saurus unit are great options at any level and can help to crush blocks that oppose you.
Magically offensive Slann in
TG are my personal favorite. These guys are going to open a can of whoop-ass in the magic phase and utterly dominate. Full Lore, Free Die, Soul of Stone and Discard 6s used in various combination are the best four for this. You will really want the Cupped Hands when taking this route. Power Stones are good options on the cheap. The unit will be a major target so the SSoC is a good banner to consider, while the War Banner is also good if/when
CC starts. You will really want support Priests to carry scrolls and the Diadem. Another thing they are great for is the Slann's Channeling ability. A Priest with the Cloak of Feathers is a great way to add 20" to a
MM range. A fly-Priest should almost always be considered when taking either offensive build. Channeling also helps when a EotG is in the way as the Priest can just be used as a vessel for the Slann allowing you to keep the EotG close for that 5+ Ward and hammering power! A decent one of these will run you 400+ points with ease and with the cost of the
TG you have a lot of punch in one unit, just don't let it die!
Defensive in
TG is okay if you want to put other points into other things, not magic. Slann are easily capable of shutting down a magic phase by themselves and with a little help, it can be assured.
MR(3), Discard 6s, PoD, the Cube, the Diadem and the Curse-Charm are all items that are going to utterly ruin enemy magic. This is pretty self-evident, load up on these and laugh at enemy attempts to cast.
Solo Slann have a pretty well set route to take, you are going to want to take Higher State and DPoP. Taking
regenerate is a good idea as well. Without
TG you are free to spend a bit more on him but don't go overboard! These toads are just as survivable as those in
TG, if not slightly more so. The set up is pretty much the same as before.
Magically offensive solos will want the same sort of Disciplines. Full Lore and Free Die will fill other spots. Cupped Hands is still very useful and should be taken. These guys suffer from not having the Large Target perk of a
TG-bunkered Slann. This can easily be overcome with an EotG and a fly-Priest. The EotG gives the Large Targets perk to
MM. The fly-Priest can give reach to other spells. These guys are competitive in a lot of ways. So if some reason you don't like
TG but want to kill in magic, this is the toad for you.
Defensive solos are the lamest Slann ever. I think of these guys coming out of a thousand year contemplation and lazily gathering some trinkets, floating out of the Temple-City (telling the
TG to have a day off) and picking on some warm-blooded wizards.. Honestly, if you don't want to focus in killing with magic, don't take a Slann. Same sort of kit as the defensive in the
TG; same options to keep him alive. He is going to cost a lot, too. If you are really worried about magic, take Priests with Scroll and have more points to spend on Saurus blocks and whatnot.
I hope this helps convince you that Slann are indeed worth taking and how to use them. Sorry about all the reading.
. The only reason I wouldn't would be if you were running a themed list (such as all skinks) in which case you could just take an EoTG or the like.
. I do remember 