Pink Horrors of Tzeentch

In a nutshell: Semi-shooty anti-infantry, in a world where everyone has 4+ cover saves and in an army with plenty of power weapon armed units. Definitely has a niche purpose if you want a shooty army, but if you really want to kill the other guy for sure--the power weapon squads seem like a better choice.

Pros: 3 BS3 S4 shots @ 18". Can move or fire, so for shooting at infantry it is quite fine. As a backup, you can (and should usually) give them a anti-tank shot. Does make the rest of the units shooting useless, but gives you more flexibility to handle transports from 24" then followup against the infantry inside the next turn. Remember you need multiple units to do any reasonable damage to vehicles due to the low strength and low accuracy. They also have a 4+ invulnerable save, which is good when out in the open.

Cons: T3. T3. T3. People, when you land this unit on the table and the other guy has so much as a IG grenade launcher--do not fire! RUN! Spread that unit out. You can be annihilated by blasts if you really want to, but you probably don't want to just give up a unit for nothing. lol

Tactics: Drop and roll (run) is my advice. Much as people want to be able to drop down and roll a bunch of dice, being less vulnerable to blast, ordnance, and flamer templates is much more important to having said unit after you pick off 3 Orks, ok? When facing a mech army, sometimes you have to suck it up and fire those bolts at max range...which is better than someone moving a transport full of rapid fire troops on top of you and unloading. You should pretty much always stay at long range and spread out as much as possible, firing at enemy infantry. You cannot fight in CC, ever. Everybody kills you. lol

Final word: A moderately useful shooting unit, not very strong in attack or defense. The 4+ invulnerable save is neat, of course you could just get a 4+ cover save in terrain. Call it the middle of the road shooty unit, able to shoot a bit but unable to CC at all.

Additional information:

I almost forgot the Changeling. If you bring a Pink Horror of Tzeentch unit, you probably want to consider him. Especially in tournament play. He can make your opponent fail LD tests and be unable to shoot. Of questionable use against most armies due to their high LD (like all the other LD bending abilities as most negative modifiers have been removed from 40K5E) but it can make your initial drop a little bit smoother against a heavy gunline army. In friendly play, you're probably already earning dick points for bringing a turn 2 auto-assault power weapon horde against your friends...so don't be a supercock and bring the Changeling too, so they can't shoot you either. ;)