My guild is a little bit behind the curve because we got our 2nd group of Naxx 10 man raiders in there just after christmas. I have been holy the whole time until a week ago when we lost our other priest who had imp divine spirit so I was going to respec anyways to lose guardian spirit and go far enough down the disc. tree to get the spirit buff. I decided ah wth might as well try out disc. for a few days at least for fun. So I respec'ed reglyphed and reset my toolbars. My first Heroic was a little shaky as I was still under the priest I can spam COH when the times get tough mentality. After one run though I got the hang of it and was doing incredible. Even on the tougher fights for mana in naxx where I couldn't get out of the all mighty FSR I was having 3/4 mana left at the end of the fight. And not using my shadow fiend like I normally would halfway through. My lesser healing wave heals for 200 less but with my new talents and gear modified has a much higher chance to crit. I changed my glyph to the holy nova increased heal by 40%. My numbers are lower but you know what thats alright with me. I know and my raid leader knows that I'm actually contributing more than I used to with this new spec.
So the moral of the story is, don't be afraid of disc. it really is a phenomenal spec just make sure everyone around you understands that some mitigation just doesn't show up anywhere. "unless you get the hacked disc recount program"