I tested it on lvl 85 with ~ 150k life. I gathered 10 mobs (were only lvl 84-85) and as soon as I dropped to 35%, I was immortal
. It was awesome. With the current mechanic of second wind, new doors just opened for warrior playstyle. This talent probably won't help you much if you are being focused hard, but it allows you to delay your death vs 1-3 opponents by quite a lot, if combined with shield wall, die by the sword etc.
It's hard to tell if it is op or fine.. I think it's fine, but we need to see lvl 90 healthpools and the amount of dmg and healing overall in pvp, when you are fully pvp equipped.