Shaman, Shaman, Shaman, Pally, Priest (3dps, a tank and a healer) -> good PvE combo
Mage, Mage, Mage, Pally tank, Priest -> Similar as above
Mage, Mage, Mage, Pally healer, Warrior -> Similar as above

Priest, Lock, Lock, Lock, Lock -> Epic damage for soloing instances in PvE(up to 50-60) with pets and possibly fun in PvP too. Locks can convert health to mana and the priest does a group heal to fill up the health again, together with those of the pets.

[edit] As a sidenote: My original choice was 4 shamans. Now I have 4 shamans + 1 warrior tank.
I still use the 4 shamans for PvP, but I will add the tank when doing instances and when he is levelled.