The changes to priests (healing priests that is, I don't know anything about shadow) in 3.1 have really turned the tables on me. I used to love the holy-spec, it was versatile and easy to use, it has a remedy for every situation. But with the new spirit-regen nerf and changes to Serendipity the holy-spec regen has really gone lost. I have about 320mp5 which isn't anything fancy. But that was compensated by the fact that I had like 900 ooc-regen with my holy spec. Every time not casting would easily regain me the mana needed for the fight (I actually only ever oomed on fights like Patchwerk). And I realize 900 ooc-regen might be a little over the top, but now after 3.1 it has dropped to 500 ooc-regen. And honestly, that is basically nothing. The new Holy Concentration doesn't make up for it.
Ok ok, I haven't been all fair, I've only tried this new regen-nerf in Ulduar (and Emalon) so far, and seeing as those fights are really, really hard (well most of them anyway), maybe my old regen wouldn't have been enough either. I am currently dual-specced holy & disc and tried both out at various fights in Ulduar some days ago.
But then I look at disc and talents like Renewed Hope, Soul Warding and Rapture. Those combined are seriously awesome. With Rapture I actually gain mana from casting the shield (if it is soaked) and mana was rarely a problem for me in the Ulduar fights we tested (the first four bosses). If I just get the timing of the shields right for the event i.e learn when the aoe comes and who's gonna get it, there is no end to the possibilities.
I'm gonna do good ol' naxx again in some days and then we'll see if it's Ulduar that has sacked my holy-spec or if disc really is the new way to go...