Warlocks are okay, better than mages I suppose.
Affliction is absolute garbage.
Demonology has trashy damage (Soul fire hits for around 125 in my gear, probably higher in BoAs, but keep in mind it alwayscrits and this is how hard it crits) but is much better at actually surviving.
Destro is a bit glass cannon at times, and don't expect to solo anything with heals as Lazertag said. If you can get a fear off on an enemy you have a solid chance of survival against dps.
Overall it's fun. Make sure you know what you're doing though.