If anyone that's made it through the first two bosses of BWD could weigh in with any tips, I'd expect this is where a lot of priests will be seeing their first Cataclysm raid encounters.
I've tried Magmaw as Disc, doing raid heals and found that I didn't have the throughput to deal with the consistent, heavy damage. I'm going to try it as Holy, but in my testing so far it seems that while my HPM while in Sanctuary should be fine, I'm running out of mana noticably faster. My "rotation" is pretty much PoM, CoH, PoH, a renew or two, yelling at people to use lightwell, and ignoring most of the other spells that have lower HPM. Will Shadowfiend, HoH and a pot be enough to keep this up through the fight, or do I need to look for more efficiency somewhere?
If throughput is your concern on Magmaw, holy is probably the way to go and having people in your raid that are 'trained' to use lightwell will certainly ease your mana consumption. The chained phases are great opportunities for regen if your group is executing them smoothly and your personal CDs + pot should be enough to last you through the fight if that's the case. I typically run with a resto shaman and have the benefit of mana tide, so my experience may be different than yours, though. I think the fight is long enough (this will vary, of course) to benefit from veiled shadows if you find yourself running low often.