Let's talk Trolls. With the new rules for Monstrous Infantry, Trolls looked like an early winner and quite a bargain compared to other medium monsters such as Kroxigors or Minotaurs (with exception of perhaps the Treekin). So with the new army book, the Trolls get a drop in points and a move to special for the common variant, while the Stone and River Trolls remained as Rare choices. Does this mean that Trolls remain a no brainer selection for all Orcs and Goblins armies? Well, yes and no.
One of the things that made Trolls so extremely effective in the old days was the combination of Trolls and the Waaagh! spell, which granted them both Always Strike First and Hatred, turning them into combat monsters. That combination is now gone, meaning Trolls will always be slow, of average weapon skill, as tough as an orc but quite a lot stronger. And of course they regenerate, which without a doubt is their main strength.
The regular Troll is definitely the best deal of the three, as they're cheap and so can be taken in bunches. Six trolls makes for a decent combat mob that will be able to go up against not-too-nasty units. However, you'll most likely feel that, despite regeneration, you will still be losing bodies fairly quickly and with that quite a lot of attacks as well. Therefore, you might want to go as many as eight regular trolls to maximise on their damage potential, and still keep your unit cost below 300. Even if you do lose a few to a stray fireball or some knights with flaming attacks, you'll still have enough to dish out the hurt.
However, Trolls don't always have to come in big mobs, and it's quite viable to use them in smaller support units as well. A single troll makes for an excellent diverter, and units of two, three or even four trolls can really turn a battle if getting in the support charge. In smaller units, you can definitely consider going with one of the rare breeds of trolls as the point increase won't be felt as strongly. You'd be surprised at how annoying a single River Troll can be, if fighting an enemy unit to their flank or rear. With their slimey hide, your enemy is going to have to devote so many more attacks at it than the worth of a single troll, lest it will remain there, dishing out hurt and winning you combats while your infantry mobs fight in the front. If you find yourself facing a lot of fire magic, Stone Trolls might be worth getting, since they still get a ward save against fire magic that otherwise will cripple a troll. All in all though, the River Troll upgrade will probably make you happier in the long run (not to mention the excellent plastic models make them an attractive buy).
As mentioned at the start, although they're quite resilient and hard hitting, Trolls do come with a lot of weaknesses and plenty of ways to die. The most obvious one is flaming attacks, whether from magic or magic items. Magic you can dispel, and there are tricks to avoid flaming weapon attacks as well. That being said, make sure you read up on what enemy units are able to bring the Banner of Eternal Flame, and if possible send a throw away unit in first to check whether or not the enemy weapons are on fire.
Also, with their low initiative they can easily die to a Pit of Shades or a Purple Sun landing on top of them. However, they still strike ahead of great weapons, which are also the type of units that Trolls want to go up against. Bestigors, Hammerers, Ironguts and Grave Guards are all units that might be tempting targets for the trolls. You get to strike all your attacks ahead of them, meaning it's likely they won't have as many coming back at you. In fact, you don't even care if they're striking at S5 or even S6, since it's likely they won't have as many attacks than if their strength was lower. While your Black Orcs might frown at things that don't allow them their armor save, as long as the enemy isn't bringing fire, your Trolls won't mind - they'll regenerate just as well.
Unit size: 6-8 for a combat unit that fights enemies straight on. Single trolls for annoyance and redirecting, while 2-4 make for good flankers.
Stone or River: If you expect to go up against fire spells Stone Trolls might be worth it. Overall though, River Trolls make for the best rare breed of trolls, in particular if you're running smaller units. Most of the time you will best of just bringing regular Trolls though.
More importantly than anything though, make sure you keep your General within barking distance, lest you want your Trolls wandering off on their own.