Trolls
For what you get Trolls are quite expensive and quite vulnerable for 40 pts. They do have some potential against certain opponents though.
With a toughness of 4, the same as a standard Orc, you need to protect your trolls and shield them as they traverse the battlefield. Unfortunately this will effectively reduce their movement rate to 4 unless you shield them with some kind of cavalry (which might not be the best use of your resources and make the 'real cost' of trolls even higher).
When Trolls do get into combat they can be useful. The trouble is, they cost 8 times that of an orc and when their unit leader is killed (which will mean the loss of a hero or, heaven forbid, a Lord) they are next to useless due to stupidity (unless they're already in combat) and their low leadership.
Overall, I don't recommend using trolls in any competitive game. They might have a use with their single auto str 5 acid 'puke' attack with no armour save against knights or heavily armoured dwarfs but that's about it.
Notes:
- If you do attempt to use them agaisnt Knights, make sure you're not charged by the Knights first !
- Definitely don't pay the extra 20 points to turn them into Stone or River Trolls because that really will make them way too expensive.
River Trolls get -1 to be hit in close combat (due to severe halitosis).
Stone Trolls get magic resistance (2) and 5+ armour save which could be useful against say, Wood Elves, but costs a lot at 20 pts per model
