Having played on test with both a level 50 eng/eng blaster and a level 10 elec/elec, I can say that the level 10 is benefitting far more from Defiance than the level 50.
At level 10, a blaster has enough hitpoints and the enemies aren't really doing all that much damage yet, so it's entirely possible for me to be fighting at a relatively low level of HP for some time and thus profit from Defiance. For my level 50 it is a different story. There is simply too much damage going on at high levels for Defiance to be of much use. By keeping my HP at anywhere short of maximum, I was risking a single hit defeat by a boss or a quick series of attacks by minions killing me and giving me debt.
I am also very concerned about the teaming aspects of this power. There are a number of posts on these boards lamenting how defenders will concentrate on healing other AT's before blasters because we die so quickly. The added HP helps some but at high levels it is still a matter of 3-4 hits killing me. I can do well when soloing, but in teams with larger spawn sizes I tend to take more damage and die more often. I am worried that my teammates will now have a REASON not to heal me and therefore I will be dying even more often. This is especially worrying with the potential changes to tanks aggro-holding abilities (such as the 5 mobs per taunt issue).
I do not think this power is the answer to blaster's woes. I will live with it if it goes live but I would much prefer to see something else.