That's something I've been trying to figure out for quite some time now. As a decent raiding Holy Priest I'm using heartsong right now but I see a lot of people running around with the Power Torrent enchant.
No matter how I look at it, it doesn't look like a good healing enchant:
-Regen-wise it seems to be much lower than heartsong which has at least a ~55% uptime.
-I simply hate +spell power proc. It's unreliable and can proc on phases when you just don't need it and then let you down when you need it the most. The only thing that makes me consider it as a healing enchant is that it's not +spell power here, it's +intell, so there's a small regen component here. Then again, if I only find the regen part really interesting, why would I pick an enchant that gives me less mana? I'd rather pick an enchant that allows me to throw 1 or 2 more PoH than on that may, if it procs at the right time, boost my healing.
Still, seeing almost every single healer around me pick Power Torrent makes me consider it as a viable, perhaps better option. It HAS TO be good somehow, right?
So, is there something I'm missing that makes Power Torrent a good regen enchant? Are other priests just gamblers? Am I the only one who actually manage to have 5k mana left at the end of a fight, WITH heartsong?