So i rolled a priest a while ago, got him leveled to the 30's as shadow hit 40 and was actually dissapointed in the way the class played. I felt it lacked survivability, some thing I often desire and was just missing something, I guess to me it just didn't feel priestly.
I respecced to a mainly Discipline build with 5/5 in spirit tap and 2/2 in the holy talent that cuts down interuptions.
I BG'd for a bit at 45+ and felt like I was missing something, the 5/5 in spirit tap basically did nothing for me so I went for a full discipline build.
Wow what a difference, improved PW:Shield is a god send, and often ends up delivering the killing blow against casters.
I start my fights off with a Holy Fire, SW:Pain, then I usually just wand until it's dead with an occasional smite. Most times I don't even need to reapply PW:Shield during the fight. In the end I still have damn near full mana and during most grinds I never need to stop and rest. When I need to I pop Power Infusion and go to town and can bring mobs down fairly quickly.
In PvP I can handle any single opponent just fine (now that I realized how nice mana burn is) and even some duo's. I even took out a warrior + shammy duo yesterday (I was 48, they were both 47). I think they were quite shocked that I didn't die. As well as a mage + druid duo, though they were 45/44 when I was 47.
Anyway, my point is that alot of people tout shadow as the only real leveling spec, I just wanted to say that it does not need to be that way. I think you just need to play to the strengths of the build you choose. I am tempted to try out a holy build as well, but right now I am enjoying my survivability :)