It's like Regeneration but with a ceiling of half hit points rather than full hit points.
It's the half hit point ceiling that, to me, makes it next to useless.
As a defender, my job is to get hit. Staying at half health is not a good idea. You want to heal up before the enemies get their round again, and if you can avoid it, not to go below bloodied at all.
Even with a 21 AC at 2nd-level in KotS, my fighter has been knocked below bloodied in just one round numerous times. Therefore staying at bloodied just so I can get a piddly 4 points of regeneration, is just silly. Next round I'll be killed.
But I can only use Boundless Endurance once per day, which means I want to save it for the boss encounter. But let's make it an encounter power, just for argument's sake. Even if I get a tick of Boundless Endurance before I get kicked in the guts again, that's what? 4 points of regeneration for the entire encounter? Big whoopy.
So maybe I get knocked down below bloodied twice and nobody has another go before my turn. Eight total points of regen just isn't that great. And if I get knocked below bloodied three times in an encounter, then the encounter isn't going very well and it's going to take far more than 4 points of regen a round to help me out.
I'm far better off negating damage altogether (No Opening) or getting 4 to 14 temporary hit points (Unstoppable) to keep me above bloodied in the first place.
In the times that I was in dire straits and needed to use Boundless Endurance, I was up against goblins that were doing extra damage with combat advantage. In one instance, it was close to the end of the encounter, so wasn't worth blowing a daily. In another, it was a boss and I used it, but died anyway because of massive damage. After getting healed, I got up and used second wind and comeback strike plus the healer's healing word to get myself back way above bloodied. There was no way that I was going to sit there and stay on one healing word's worth of hit points. I just would've dropped the next round.
In both instances, No Opening would've negated the bonus damage and I would've stayed up, and healed again to stay close to full. In both instances, Unstoppable would've given me the bonus points needed to stay up and heal again close to full without going unconscious.
I'm pretty clear in my view that Boundless Endurance just isn't that good. Sounds good, but isn't.