As a DM having seen this power in action: It's strong and very noticeable. Eve at early paragon it can still be. Daze and Stun effects are always annoying for players, so using more of it is just a kind of "annoyance" spiral. I am annoyed because my monsters don't do any damage, and react by annoying you with daze and stuns. I don't like it.
The Cleric in my campaign is actually using 3 first Level Daily powers at early Paragon Level, each giving significant boost to healing or resistance. I don't know remember their names now, unfortunately. It means he is even _really_ prepared for a single fight, or he can cover 3 encounters a day, which is usually pretty much the number of encounters you might get anyway (that is part of the campaign or play-group dynamic. We don't play as often as we used to, and expecting to remember which powers were active 2 months after the fight is a bit much. Notes get lost.)
Edit:
I also seem to remember the character has an item that can give you back a single power as a daily power. But I don't know if this applies to this power or one of his other abilities.
Possible Change
This is bound to collide with some other powers, but one change could be:
Until the end of the encounter, the target gains 5 temporary hit points at the end of its turn.
This will still negate a lot of damage, but not all of it. (The end of turn limitation ensures that this isn't a must-have for anyone that has to start his turn with temporary hit points for some kind of benefit.)
Alternatively:
Until the end of the encounter, the target gains Resist 5 while bloodied.
This kinda violates the spirit of the power, as the "moment of glory" only applies when you get seriously injured, but it means that players still take a significant amount of damage, and it encourages "gambling" - stay bloodied to take less damage, hoping no one will hit you too hard. The time a player character is bloodied, a lot of minions might already be dead.