When you get high enough level (~lvl 50 or so) speccing destro works quite nicely - make your priest shadow for mana regen, and just burn down the mobs. I find afflictions locks (as a pair/group from experience of playing with them) too slow and mana-ineffcient.

My $2c. Remember to drainsoul most mobs as you kill them as you burn through shards like nobodies business as destro