Can doa lot more with shaman - heroic instances, arenas, etc.

However, warlocks are definitely fun in world pvp, bg's, and for farming. The dots are brutal. With my group, in crap gear, I can kill almost anything with just 2 sets of dots if they don't bubble or heal. With all 4 dots cast, it's over. Being able to do damage on multiple targets at once is another nice benefit. Works equally well in farming - you can pull a dozen mobs at once. Or, with siphon life and 4 warlocks I imagine you could pull forever with life tap.

Really depends on what you wanna do and your play style.