Warlocks are tough to play unless you totally power game feats and invocations (Such as Mortalae feat and Hellfire blast invocation)
If you play them straight from Complete Arcane, then its not a whole lot of fun. Invocation growth rate is slow as all heck, which can make them borning as well, doing only a few spells over and over again.
6th level is when you get all the fun invocations..and you break loose all hell.
Warlocks get a small amount of DR and fast healing, but can die easily in a fight having little AC and HP. Take Elderic Spear and stay far back!
I have a Warlock in a campaign right now, and am really enjoying playing him because its more high end role play any way.
---Rusty