In 25 man, it's usefull as a tank cooldown with Inner focus or Spirit Shell. That's about it, really. Last full clearup I did on my priest, I used it 10 times. Half of those were Malkorok (topping off shields), and the other half I'd imagine would be for tank coverage. PW:S for borrowed time on tank (+ buying him another hit) into greater heal is more than 1M effective healing done in about 2 seconds. If the tank is low enough that he's killed off within the 2 seconds (with a 400K shield being thrown on him instantly), your other healer(s) failed worse than you did, or your tank did something wrong. Nothing hits that hard, that quickly anymore.
greater heal is only useful for holy pretty much and the very rare occasion for disc but i have rarely had the need to use it on disc. But 2pt16 greater heal is very nice for holy, with wings it can crit for over 800-1mill normally.
I use it infrequently, but not rarely. Examples being combined with inner focus for tank dips (When solohealing the upstairs on Dark Shamans, Shield Bash on Paragons, Malkorok, Iron Juggernaut) and combined with spirit shell if there's pretty high tank damage going on. If there is high tank damage and the raid damage is moderate, I sometimes like to use 2 PoH Spirit Shells on the raid and one GH Spirit shell on each tank. Other than that, penance does a better job alongside atonement.