Apparently it negates 100% of damage if the attack would have done more damage than your MAX HP (not current HP). It also has a 2 minute recast and 1 minute duration.
Basically, epic fail unless you're doing some hella scary giant NM. Useless for most day-to-day solo-type stuff.
I'd say that's pretty much win to me.
At what point in "day to day soloing" would you even be worried about dying? When we could already solo stuff like Serket, Kruetzet, Bune, or even Ash Dragon [albeit with meds and slight difficulty on that one] at 75, anything not with a deadly TP move is pretty laughable on NIN.
Tanked Iratham yesterday on NIN/DRK 2500 or so HP, ate 2200 comet no problem, while the PLD died [yes yes lol PLD, they were using him to hold for proc !!, but wasn't really needed as I also had hate from 95% to death.] Ate a 4k tp move on a chariot in A-Grauberg and got rocked... This move is win personally.