And even then it's not really worth it. You basically take half the damage you inflict (if you wear the mask).
That's pretty bad, especially when a fair few enemies have more health than our characters.
Hm.
Hp is 100 + (25 * level) + (toughness * 5) + (presence * 2) + bonuses (Champion's Resolve and so on). Max stats are level * 3 +2.
So max HP at level 6 would be... 100 + (25 * 6) + ((6*3+2) * 5) + ((6*3+2) * 2) + 20 = 410. You inflict a third of that with Exertion, so 135 unavoidable (nice!) non-crit (boo!) damage to a single enemy while also dealing 82 damage to yourself.
Yeah, the ability is really not good. Even a build fully invested in pumping HP does kinda mediocre damage with it (you need two shots to kill a single enemy, normal single attackers are better and AoE deals more total damage).
If it was enough damage to one shot a weak mob (or two shot a strong one) I'd consider it at least because that'd be a niche but as it stands the ability isn't worth the slots. Even worse, the other At-Wills are far superior especially with stances now being automatic.
Levels are supposed to go to 10, so max damage would be 196. I wouldn't be surprised to see standard attacks doing more damage at that stage.