Divine Sacrifice has been redesigned - Damage which reduces the Paladin below 20% health will break the effect.
So that solves the question of when it will break. All the raid needs to do is keep their Paladin above 20% for the 10 seconds it's active and you get a 20% damage reduction to the entire raid.
Personally, I don't tend to use Divine Sacrifice very often as I'm usually main tanking and it's more of an offtank skill. I'm spec'ed into it to get to Divine Guardian for longer and more powerful Sacred Shields since we don't run with any Holy Paladins anymore in my Raid group.
However, Raid Leaders have found Divine Sacrifice to be a very powerful tool for certain parts of certain encounters like Anub'arak Heroic when he's in Phase 3. Matticus of World of Matticus told us on Friday night that all his Rets and Holys are speced into it.
Now I had meant to get this into my Monday Shoutout post, but I forgot.
I also neglected to add this.
Recently, Paladin Schmaladin talked about a rather controversial issue on her blog. I'd link it, but the picture is NSFW. I thought it was well done. I had wanted to comment, but couldn't get my thoughts out in way that I felt communicated how I felt well enough.
Then I heard this talk on a similiar subject. I thought it was well done as well, and expressed my feelings much better than I could.