Preface: I am a deep-Discipline priest in a Black Temple guild. I've been using a 40/21 Discipline/Holy build for raiding and it has worked to my satisfaction. Around New Year's, I respecced to pick up Pain Suppression and see how it fared in a raiding environment.
I've been raiding with PS for a few weeks now. Here are my thoughts.
It's too hard to cast under many circumstances. The circumstances under which it would be best used move too quickly to get a PS off. Most times someone pulls, he is killed (or taunted off of) before PS arrives. So the damage mitigation is much more situational than I'd hoped.
There *are* uses for this spell in two sorts of situations: aggro-limiting, and aggro-independent.
1) In aggro-limiting situations (i.e. those where everyone is running against the tank's aggro generation the whole fight), PS can be successfully used to help someone shed aggro. Hybrids in particular benefit from this, i.e. boomkin druids, enhancement shaman, DPS warriors, etc.
Unfortunately, using it in this way means taking our eyes off the health bars and onto our Omen displays. It also doesn't prevent the DPSer in question from pulling aggro anyway. (I cast TWO PSs on a certain Enhancement shaman during our last Loot Reaver kill and he _still_ pulled.)
Furthermore, aggro-limited fights preclude using PS on the tank. Unfortunately, experience has shown that our DPSers track the tanks' threat extremely closely. That is to say that almost every fight without some sort of Fixate mechanic is aggro-limiting. Because DPSers track so closely, in many situations a PS on the tank would guarantee a wipe.
The only way to avoid this scenario would be to monitor Omen closely enough to know that a PS wouldn't cause a pull. Experience has shown it's easier and faster to try and get a heal off. The fact is, I don't have the reflexes or the discipline to see an emergency, glance at Omen, guage if the tank is in the clear, and get back to the tank with PS before a) he's safe or b) he's dead.
2) Aggro-independent scenarios. These are fights in which only a tank (or designated person) can possibly have aggro due to a boss' mechanics, i.e. ranged cannot pull Al'ar phase 1, Netherspite only attacks the red-beam guy, and so on. PS can be very handy for use on tanks in those situations because the anti-aggro aspect of the spell is irrelevant. Unfortunately, these fights are few and far between; plus PS, as a reactive spell, has trouble enough in emergencies.
There are two Black Temple fights that use this sort of mechanic. PS looks like it could really shine there. The first is Gurtogg Bloodboil, where Fel Rage overrides second-stage aggro concerns and the damage output gets absurd. The second is Reliquary of Souls, where Fixate governs phase-one behavior and healing is impossible. (Plus, RoS also has an aggro-limited fight with AoE damage, where PS could allow one DPSer to shine like the sun.)
Therefore, in anticipation of facing these fights again, I'll keep PS for now. But I've been sorely disappointed by it. Intellectually I knew the downsides to taking it, but I thought the upsides would be enough to justify giving up Improved Death. So far, that hasn't been the case. Most of the time I just give PS to whomever I PI... which is nice, but not worth giving up Holy Form and 50-odd Spirit.
In fact, the talent that has been intriguing me more and more is.... Circle of Healing. I know its HPS isn't the best, but have you seen how many of these fights involve AoE damage? Have you seen shaman dominating the charts doing nothing but Chain Heal? A mana-friendly out-of-group heal would be a significant upgrade over trying to Flash it all.