You can check out the Priest forum on WoW community site. Most of the feedback I've seen is that it's great if you solo. However, undead no longer flagged undead so you can't control them as you used to in PvP situation.
One big down side I've seen in the posts is that if a priest is shadow priest, his/her mana pool will be smaller due to lack of the bonus from holy tree. They said it'll work out fine at lower levels, but if you want to do high lvl instance (50+), it's best to go as a holy priest.
I really think it all depends on players' skills though. There's many way to play a game, holy priest is just a more common way and easier way to do it.
Soloing is definately harder for a priest than a mage. I miss my gnome mage already.
Shadow Priest discussion