I have a priest myself, and when i first looked this talent all i saw was "sacrifices self for 50% of target Hp." I thought to myself that this talent isnt really all the great.
I came back to it later then i saw the first part "Increases healing recieved by the target by 40%." Thats when i decided that the main point of this talent was to increase healing on the tank at pinch times. Trying to conserve mana or the healers are low? throw this up and you wont have to send as much mana toping the tank off because they are reciving 40% more.
tested it out on a random person standing around Shatt one day. without Greater heals hit for about 4.5k, crit for 7k. with it on, they hit for 7k, crit for 11k.
All in all, i think everyone is looking at it the wrong way by getting the primary purpose mixed up. Its 40% healing with an "oh sh!t" built in.
"Ive always wondered what we warriors say to make people attack harder, be healthier, or taunt mobs" -Orla