This stacks with Armor of Agathys really well. I'd find it well worth taking as a warlock now.

I'm not sure if it tips over into the point of being abusable. My gut feeling says no, but I think I'd have to see it in action to say for sure.

Being able to cast it on allies opens up some fun possibilities, too, especially if you've got ways to encourage foes to target those allies with their attacks.