You'll love 5 Warlocks until you hit the TBC dungeons.

My first 5-box group was 5 Warlocks and we steamrollered everything that vanilla WoW threw at us but just couldn't do the last boss in Ramparts (dragon + rider, forget the name) with them, they needed healing... swapped out one lock for a priest and started steamrolling again

I now drive with the priest, using Healbot to keep the locks and pets up and it works really well.