My Priests are both Holy with a spattering of other tress as necessary. Always have been and always will be.
My Horde Priest is one of my 70's and he had absolutely no problems leveling and only started doing the Burnig Crusades instances when his guild mates needed 'healy' types for the run. He did not gain levels quite as fast as my Hunter and Warlock (who are my other 2 70's) but then he never did.
Someone above said that Holy Priests have no damage spells but there are many a dead critter in the wake of my priests that would beg to differ on that point. When some critter jumps my priest and is looking for an easy kill they normally are quite surprised when I layon my shield and then absolutley unload on them. When soloing I start with the Holy Fire ( I believe it is - long cast time DoT) then while the critter is running towards me I plop on my shield and hit him with the other DoT whos name I forget, and that start in with the mind blast and the basic one again whos name I forget.
Then if the guy is at or above my level I will replace the shield and then just lay into them with my Wand. If I am doing guys 1 or 2 level below me I can usually do in three before needing to drink (they both carry three stacks of the best water they can find and after 60 usually had to replenish them during a normal 3 hour killing spree). Critters my leevl and 1 or 2 above I am lucky if I can kill 2 before having to drink.
I think the poor rep of Holy Priests not being able to solo well is 100% due to them not killing stuff rapid fire like many other classes can.