I've been playing a Priest on the PTR (Disc, mostly for PvP). It's pretty simple: there's no reason to pick Spirit over MP5. Spirit has become the poor cousin of mana regen, and yet it's still all over my gear. Feels like an utter waste. The alternative is to pick the +crit set, but that's not exactly the most useful stat for Disc either.
Now, I'm not suggesting they start adding +MP5 cloth sets for all classes, that'd be silly. I just wish it did more for Disc Priests. I don't like being balanced by making my gear inefficient.