Playing a Disc Priest has been incredibly depressing. On Feng (25m), I was consistently the last on the Healing meter (including absorbs) and Overhealing meter, and the first one to go OOM / the healer with the most mana problems. I was able to output more healing back in T13 than I am able to output now in 468 gear (or at least about the same output). I spend maybe half of a fight, if not more, either DPSing the boss doing 8-10k DPS just to TRY and get mana back or just plain OOM. If it's not obvious, it is incredibly inefficient to DPS a boss as a raid healer, not to mention Solace is just awful in terms of mana regen. Disc used to be fun. I was a huge advocate for Disc in my guild, and hated the idea of going Holy. Now, I can't even find Disc as viable for certain fights.
To compare:
Disc Log VS
Holy Log
In that Disc log, I casted Solace 50 times (67.5 seconds in a 6:26 minute fight just casting Solace) and got 109k mana back. Over a minute of doing no healing just to gain that little mana.
Disc is in a horrible place to say the least. Our regen went from one of the best to just plain awful. I find Holy to be in a pretty good, strong place right now, with basically no real change in healing style from Cata.