Flesh Hounds of Khorne

In a nutshell: One of the best Fast Attack choices available to the Daemon armies, if not the best. If you just have to fill the FA slot, this is a very good unit to bring along with all the goodies they get. Speaking of goodies...

Pros: 3 attacks on the charge at S5 and I5. Beasts, which gives them mobility the Daemon army as a whole severely lacks. Of all the fast attack choices, this one at least has the virtue of being better than 'crap' when compared to tactical marines, right? They also come with a 2+ save against damaging psychic powers, and what's really amusing is this also gives them a 2+ save against force weapon hits. Not a force weapon attack (psykers can't execute one on Daemons anyway) but it makes the default weapon of all psykers utterly useless against these (literally!) demon dogs from hell. :)

Cons: Still only T4 with a 5+ save. Not exactly survivable. Better than the other white meat, the Seekers of Slaanesh, but they still die to rapid fire or concentrated heavy weapons fire. Which leads us to how to avoid this...

Tactics: While I don't recommend more than one pack, you can really make one pack an excellent unit. Still pricey for what it does compared to other Daemon units, but having a fast beasts unit is invaluable. Ask any Tyranid player how useful they really are. So, to make this one pack really good; add Karanak and upgrade another model to rending capable. This gives the whole unit Move Through Cover, which combined with Beasts movement is truly a unit that can get through cover quickly, which boosts their survivability. I'd recommend a squad of 15 to make it difficult to get rid of Karanak. Without him, you lose one of your renders (the better one) and move through cover.

Final word: While most people don't really believe in sacrificing units that consist of 15 point models, this is really what this unit is for. Making it better able to do it's job and hang in until the rest of the army arrives isn't a bad thing. You can play hide and go seek with your plaguebearer/bloodcrusher armies with mobile enemy armies if you really want to, but you won't like how that works out most games. Having something that can reach out and lock down a enemy unit, even pull enemy models towards you, and give your normally slow infantry models a free assault move forward the next turn...that really is priceless.