Obviously Power Word: Shield is going to be awesome come patch 4.0.6, if it doesn't get nerfed into the ground before then. I uploaded a fragment of a spreadsheet I have to google docs so you can see some of the numbers yourself. Check out the bottom of the "Spells" sheet here. The values on the sheet are based on my current gear.
For those who don't like looking at spreadsheets, the key facts are that Power Word: Shield has 97.2% of the throughput of Prayer of Healing and 73.3% of it's efficiency. Those are some pretty strong figures. The only single target competitor is Penance, which is 7% more mana efficient than Shield, but Shield has 72% better throughput than Penance. Nothing else in the single target department is ever close to Shield in either category.
The question is, is shield really a single target heal? If there was a healer who had an instant cast heal-to-full spell that didn't cost mana, but couldn't do anything else and that spell had a 15 second cooldown, you probably couldn't afford to use one of your three healer spots in a 10-player raid on them. 15 seconds is really a very long time to wait between being effective. Of course Discipline priests can do things when they are not casting shield, but the question is, is it right to consider a heal that can only affect a single target once every 15 seconds a single target heal, or is it more like an AoE heal?
As an AoE heal Shield is right about where it should be. Worse than Prayer of Healing in raw numbers, but probably better than Prayer of Healing because of targetting restrictions and overheal. Prayer has a tendency to overheal in excess of 20% because it will hit a target who doesn't need the healing. Shield doesn't overheal much if at all when used correctly.
Shield is both better an worse than other AoE heals. It's better because you can stop the damage from happening at all and react quickly and powerfully to individuals who are at low health. It's worse because you have to anticipate damage rather than being allowed to react to it, and because it doesn't heal evenly over multiple targets.
The second part is a minus because AoE heals are much better than single target heals. If a big AoE is coming at five people and I shield two of them before the AoE hits then those two people don't end up down health, or at least not much. If I then choose to use Prayer of Healing, my best spell for healing three people up after an AoE then it is working at 60% effectiveness. My pre-shielding was acutally just pre-overhealing. If I can shield all five people, or shield four and use Penance on the last one, or if the damage is dangerous enough that waiting on Prayer to heal them up after is too slow, Or shield everyone but the shadow priest and the feral druid who will heal themselves eventurally, or shield the one person who isn't in the same group as the others then Shield was great. If I just shield a couple of people at random, Shield may end up being a total waste of mana.
This is also why just casting shield every three or four seconds probably won't contribute enough to the group healing. By keeping a larger number of people at full health through the AoE you are denying your co-healers maximum mana efficiency in order to maximize your own. With smart shielding this shouldn't happen, but there are some damage distributions where it's simple not possible to contribute beneifically with a single-target effect that cannot be repeated within 15 seconds.
Back in Wrath I read all about how Discipline priests did nothing but spam Power Word: Shield. Some players countered that they cast all kinds of heals and that people who just spam one spell aren't playing right. The argument from Blizzard was that while spamming one spell may not have been the best way to play, it was close enough to the best way to play that it was a problem.
This never entirely clicked with me, but I realized at some point it was because I raided with 10 players. With a 1 second GCD on shield spam and only 10 people to target every 15 seconds, I was forced to do other things. Not that I kept a shield on everyone at all times, but even in a fight like Freya + 3 where putting shields on everyone is the norm, I still had to spend some of my time on something else. Now that the majority of raiders are raiding 10 player we may see shield "spam" go down dramatically even if Shield is substantially overpowered.
If Power Word: Shield is an AoE heal, then it probably balanced. If it is a single-target heal then it is grossly overpowered. In reality it is neither of these things, and I think the experiment of making it extremely powerful will be an interesting one. We'll see how it works out over the next month or so.