Alright, so my lock is only level 37 but I absolutely love my choice of class.  Yeah I know, I don't have all the experience that some of the other posters have but I love playing a warlock.  I've done ok in PvP so far but it gets better all the time.  It isn't easy (it would get boring fast if it was) but come on, warlocks have too many nice toys to say they suck.  Oh and from my experience so far is that cloth, leather, mail, it makes no diff because your aren't supposed to let them hit you anyway.