I'm leveling one of each at the moment (should be lvl 30 today). I have a pally tank, a hunter as my number 2 (backup tank, DPS and pet for off tank) and a warlock (DPS but with pet for 2nd off tank or even to drain tank, provides CC for certain types) and mage (DPS, CC, taxi (at lvl 40) and water supply). I have a priest healer although with my pally tank I mainly have the priest DPS, but if things go west and I switch to my hunter to tank, then the priest earns his keep.

The "self buffs" are insane. Not sure how affective this combination is but it's amazing fun!

I used to like my 4 druids for the flexibility (and 4x CC is amazing) but I really think you need some pets for off tank so when thing s get complicated you can at least put part of the fight on "auto pilot".

My team mate Alex is running 2 pallies and 3 locks and is having a blast - literally!