i started playing mine at 60 (RAF granted levels), so don't know much about lower levels.
At 60 however they were great fun, and could easily take elite mobs 3-4 levels higher solo (like the colossi in Hellfire peninsula)
Exorcism / Crusader strike / Divine Storm for example are all great ailities that you get as you level, and make the game fun.
At lower levels most classes are a bit boring as they have so little to play with (sub-40 Enh shaman or Shadow priest for example)