-I resubbed four account this weekend. All my experience is based on about 6 hours of AV at level 79
-My gear sucks, even for PvP at 79. Experiences likely will differ with great gear
-I was able to test shadow with 4 priests, but then 2 dinged and was only able to test disc with the last two
-the massive healing nerf to holy nova sucks bad. I am healing my other toons for about 400 each now. When I looked at my old video I was healing my other toons for 1k+ each... holy nova may not be viable for keeping anyone alive unless they buff it back to usefulness

I started with shadow thinking that was the way to go for Cataclysm. Here are my initial reactions:
-shadowy apparitions aren't that useful, even when multiplied by 4, for PvP
-mind flay doesn't cut it for PvP as the burst just isn't that high
-mind spike (level 85) could change everything, but until then shadow is weak
-I was able to set up a "CC button" that would practically allow me to psychic horror, silence, and fear on command. With four priests you can basically do this whenever you want, which is very nice. However, it would take a lot of practice to be good
-Summary: with practice this team may be ok, though going to have to figure out how to survive without holy nova. However, we'll have to wait until mindspike is ready, because using mindflay just doesn't do enough damage quickly enough.

A little bit out of frustration, I switch to disc. Here are my initial reactions:
-until I get mindspike, there is no question that I will be playing discipline... it was fun!
-penance hits hard, and I'm guessing 4x penance will be a nice source of "burst" damage every 10 seconds
-though I'm still crafting my way to cast PW: Barriers on demand (95% of the way there) I can see it's usefulness. It really does help and I feel "safe" as long as that barrier is there. With the proposed two minute cooldown, and four to use, at least one should be available for every fight
-holy nova still sucks, but at least divine aegis helps a little


TL;DR - disc is fun, shadow is gimped without mind spike, holy nova nerf hurts bad... real bad