I'm a Black Temple raider. My guild is disproportionately heavy in priests, and we have a few different specs, so we chat a lot and compare notes. I decided to post this in order to help other priests out, and provide information about what we've seen.
This post is about *healing* priests. The usefulness of shadow priests is undoubted and the format of their builds is well-established. Therefore I shall say no more about our shadowy brethren.
There are three basic priest specs for end-game healing: 41Disc/20Holy (deep Disc), 23Disc/38Holy (cookie-cutter or Holy/IDS), and 20Disc/41Holy (CoH). There are some slight variations in the exact talents taken but those are the major formats. Now I'll discuss them individually in more detail.
HOLY/IDS
This spec provides very strong healing power to the priest's staple heals as well as a nice raid buff for all casters. Endurance comes from Holy Concentration, Spirit of Redemption, IDS itself, and the ability to cast fewer heals due to the higher power.
This spec is extremely potent in early raids and five-mans. It's hard to knock it, as it has no real weakness. If there is a downside, it's that an additional priest of the same spec adds nothing extra to the raid aside from more strong, enduring healing capability.
DEEP DISC
This spec is lacking in healing power compared to either of the Holy builds, making up (to an extent) with extra endurance and two nifty cooldown-based abilities: Power Infusion and Pain Suppression. Definitely not a spec for meter-whores; however, since it packs IDS it does much to complement a second priest with CoH.
This spec provides the raid with capabilities it otherwise wouldn't have, and with a competent raider at the controls the meters shouldn't suffer too terribly much. The trick to making this build worthwhile is in proper use of the cooldowns. In many fights it's worthwhile to shoot someone a PI as soon as practicable; other times you need to wait for it. An example is the Kael'thas fight. Saving a PI for a warlock during the AoE against the weapons is WAY better than blowing it against an advisor.
Unfortunately, one of my favorite uses of Power Infusion is no longer viable. Casting it on a prot paladin no longer provides the insane aggro boost it once did. So the build has taken a bit of a hit.
Pain Suppression, being a reactive ability, is much trickier. In many cases, it's desirable to simply drop it on a high-threat DPSer, possibly at the same time as you give out your PI. In certain fights this ability positively shines. Gurtogg Bloodboil is made MUCH easier, because you can PS a person who otherwise wouldn't survive a Fel Rage and make it far easier to heal. The Reliquary of Souls fight is also easier with this ability, which has valuable uses in all three phases of the fight. And it provides an invaluable extender to the Kael'thas fight, since it allows your tank to survive a Pyroblast that would otherwise one-shot him while your DPS knocks down the Shock Barrier.
Unfortunately, this spec still struggles overall because, even in the Bloodboil and RoS fights, Circle of Healing exists.
Side note: In the past I used a 40Disc/21Holy build to maximize my regen. My analysis shows that this spec is no longer needed. Potting regularly and using cooldowns/trinkets smartly, I don't really run out of mana any more, even with a ludicrously inefficient healing routine.
CIRCLE OF HEALING
One of the most lambasted talents when originally announced, Circle of Healing has since proven highly desirable in certain circumstances. While not useful in all situations, Circle of Healing shines in certain fights, most of which are in Black Temple (although there are a few in TK). The spec loses Improved Divine Spirit, which is a hit if the raid has only one healing priest; however, CoH is very powerful in certain fights, such as Najentus, RoS, and Bloodboil.
CoH still has its downsides, primarily its small radius; targeting is, in fact, the major effort involved with using CoH. Any fight which requires lots of spacing hurts CoH. Early raids tend to lack AoE damage, so it's not terribly useful until Tempest Keep. It is similar in its overall function to Chain Heal, so if you have a strong cadre of Resto shaman then CoH is a bit superfluous. Still, CoH is no longer the black sheep talent it once was.
FINAL THOUGHTS
Holy/IDS is still the most solid overall priest healing build. However, Circle of Healing builds are proving their worth in high-end content, and Deep Discipline adds some interesting tricks. If I only had one priest, I'd want a Holy/IDS priest; but if I could have more than one, then some diversity becomes intriguing.