I'm currently lvling up a pally/3xlock/druid group but also have a 70 priest. In PVE druids are ok.. but they require a bit more micro management than a priest. Lifebloom is great but you have to keep recasting if you want to keep it flowing. In instances the lack of aoe hurts a bit.. you can still get through but it can get pretty crazy. With my priest as healer it's easier as you just hit one key and get aoe heal. Also due to recent spirit changes priests benefit quiet a bit in terms of mana regen.