Personally I dont think shas'o are "situational" at all, for the additional 25pts, I think theyre golden, +1W allows them to soak bolter fire and such that little bit better, +1A and +1WS actaully makes them as you said rather decent in assault, if that 1W
SM character manages to get into combat with you, then you've actually got a pretty good chance of beating him. The +1LD is brilliant, as tau as a whole arent a great army for leadership, and making the battlesuit team hes accompanying ld10 instead of the 9 the shas'
el would is well worth it. the +1BS speaks for itself, as on those occasions where markerlights are used on a different unit, hes on his own, or your marker units have been killed, hes still highly accurate and able to toast people. I run my shas'o with a plasma rifle and ion blaster, and hes able to put serious hurt on
MEQ and
TEQ, and characters, which is what you want him for
Stealthsuits
imo are pretty useless for games under 2500-
apoc level games. In
apoc theyre useful the rapid insertion force, but at smaller level games theyre a waste of points in my experience. Its a shame because I love stealth suits and have a lot of them (the old XV15's, I use XV25's for characters and blasters) for the 30pts, 3 fire warriors are more worth the cost, and dont take up the elite slot which is very valuable for tau. To sum it up, cool, but too expensive.
Kroot are the same as stealth teams in my view, the larger the points the more they have use, but at the average game size of 1500-1750 they just arent worth the cost, fire warriors are more survivable and hit harder, despite being over costed. Kroot shooting isnt much better than guardsmen shooting but is way more expensive, and they got mauled by stand and shoot and then due to low I and aweful armour save (if any) they get mauled again in close combat. People use them to "hang on" to objectives, but for +3pts its more worthwhile to take fire warriors.
My basic summary for tau:
Shas'o or shas'
el, both are very useful, shas'
el for a cheap leader with decentish stats, shas'o if you want a powerhouse, but more expensive
Bodyguard teams - not worth it until larger point levels, even then not really that great
ethereals - not worth it, only real use is for preferred enemy shinannigans with 2 of them, only for
apoc really, and very meh
Special characters - useless except for O'Shovah who's still not very good but at least can do something
Crisis teams - Best unit in the army on par with broadsides, dont spend too many points on upgrades though or you end up with an expensive unit thats really got the survivability of 6 space marines
Stealth team- see above
Fire warriors - Overcosted unit, but better than kroot, you can go with the bare minimum 2x6 and spend the rest of your points on heavy hitters with the hope that you can level the enemy enough to win 1obj-0 but i personally find myself taking 3x8 at the moment, sometimes with shas'ui, but thats very debateable if its worth it.
Kroot- see above
Pathfinders- Now these are your workhorse unit, tau are all about synergy and pathfinders are unit that starts it all off. Markerlights are needed to make units like crisis suits work, depleting cover saves on enemys to annihilate them with weapons that ignore their armour is brilliant, as is BS5 on units. Seeker missiles are generally a waste of a markerlight however. The problem with pathfinders is the "devilfish tax" which brings me on to:
Tetras - Now, these are a forge world unit found here in forge worlds update to codex tau empire:
http://www.forgeworld.co.uk/Downloads/Product/PDF/t/Tauupdate.pdf Tetras are ideal since they dont have to pay the devilfish tax for the markerlights and can have a targetting array. So long as you keep them moving they have a 3+ cover save from all attacks over 12" away which keeps them alive, but they are AV10 with 2 hull points and open topped, so if you are'nt careful they will die even faster than pathfinders. A clever opponent will kill your tetras and pathfinder first, which will more often than not cripple your army, so manoeuvre your units carefully to block line of site to them as much as you can and keep them out of range. One last thing about tetras, is that due to being a forge world unit, some people think that they are "not playtested" or "inbalanced" personally I think these people are not worth playing against because they are more often than not
WAAC players who will take any excuse to get a one up on you if they can. But basically just check that people are ok with them and let them see the rules so they aren't taken by surprise.
Gun drone squadrons - Not much to say on these, not really great, not aweful, very meh unit that doesnt tend to fit in most lists, cheap, doesnt do much damage, almost never taken. The oppotunity for pinning is there but pinning is so unreliable its not that useful. One use for them however is deep striking behind a tank line and hitting vulnerable rear armour or enemy heavy weapon teams where their cover cant protect them, but still not a great unit.
Piranahs - Pretty much a one trick pony unit, they either kill something and then die, or just die. These and vespid are the units I never use. Theyre not really worth it from what ive seen though, since the only decent use seems to be blasting up to a tank and hoping to pop it with a fusion gun before it kills them
Vespid - Possibly one of the worst units in the game, and
id say the worst unit in the tau codex, expensive, weak, bad armour, short range guns. Pretty much pointless, which is ironic given their high points value
Broadside teams - Brilliant units, a twin linked railgun is ideal for killing tanks and aircraft thanks to the re-roll. Targetting arrays or advanced stabilisation systems are great upgrades for them, and a team leader with a blacksun filter doesnt tend to go amiss for +8pts. PLasma rifles are generally not worth buying, but occasionally have their uses, as rapid firing and twin linked can occasionally level a terminator squad that comes down near you. Smart missiles are normally the better choice since you'll be blasting tanks all day long anyway from across the board. Also use cover and stay back, broadsides attract a lot of fire and only t4 means they get instant death'd or just blown away by massed fire if you're not careful.
Hammerheads - A bit over costed, but so long as you give it a disruption pod and keep moving very survivable, railgun is the weapon of choice as the ion cannons job can be done by crisis teams or snipers for cheaper. Multi tracker is often worth it for the 12" move and still able to fire at normal
BS. Burst cannons are normally better than smart missiles, as normally the railgun is the only gun you'll be shooting anyway and once the enemy gets close the cheapness and more shots of the cannons win out.
Sniper teams - Generally not really worth the heavy slot they occupy, which is a shame because they are'nt actually that bad. Maybe a little over costed, but they can prove useful vs units such as bloodcrushers, thunderwolves,
IG weapon team, devastator squads, and lone t3 characters. Use of markerlights to remove cover is very useful with them.
Sky ray - Was semi useful until the new
FAQ which made seeker missiles hit flyers on a 6, now they're worthless and only really come above vespid in the codex.
Just some notes:
Blacksun filters are brilliant, and only 1 model in the unit requires one for the whole unit to ignore night fighting, worth spending on most characters with armoury access
Targetting arrays are nice on crisis suits, but expensive and means you wont be firing two weapons. Normally a better idea to let markerlights do the
BS increasing.
Disruption pods are brilliant, take them on all vehicles
Devilfish are overcosted, but have their uses of protecting fire warrior teams with their 3+ save from disruption pods and jink, flat out makes this a 2+, perfect for moving 6", unleasing
FW team and letting them shoot, then flat outing to block
LoS to the fire warriors
seeker missiles are generally not worth it, markerlights have better things to do, and 10 points for a single shot crack missile that uses up a marker and still might miss is generally not brilliant
Plasma on crisis suits is how you kill terminators
Fire warriors in larger squads in rapid fire with BS5 from markers is a pretty effective infantry rape unit
Missile pods + plasma is probably the most common crisis loadout, effective vs heavy infantry and light vehicles at medium-close range.
Plasma+ Burst cannon on crisis is good vs Infantry and heavy infantry, large amount of shots coupled with markerlight hits can prove devastating, especially at rapid fire ranges.
Shield generators are generally not worth it except on shas'o. shield drones however are brilliant, and should be placed in front of the squad, use the jetack moves in the assault phase to position units in the best way to recieve fire, drones at the front, characters safe, wounded models at the back
Gun drones lack the invul save and the better armour, but are cheaper and add ablative wounds to teams, only really useful when you take a crisis team with twin linked weapons and drone controllers for 6 drones, which can take hits for the crisis team. Remember that drones count towards 25% models lost morale checks though
Probably a lot of stuff ive missed, hopefully thats helpful. Remember this is just my view, but I've been playing tau for about 10 years, so hopefully my opinion counts a bit
good luck playing tau, theyre a fun army and look awesome
3k
2k
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1k
1k 
RAWRR! 