Aye, spirit cabbies are astonishingly good at leveling quick, almost as good as a necro (better in some respects).
It's fun to see a fire wizzy drop a yellow/oj mob in 2 DD's, but I've noticed that in RvR, a lot of people are resistant to heat. I have heard that earth wizards are better for RvR.
Theurgists are cool for taking keeps with... I was zerging in Emain the other day, assaulting the supply keep with no real player resistance. It was fun to be able to sit back and let the 50 theurgist in my group stick about 15 earth beings on the door and watch it drop like a rock. Easiest keep take I've ever been on.