HOLY CRAP AND A HALF!
Wow, this isn't called Ultimate for nothing! Given the filesize, that's a huge amount of gameplay packed into one little Flash game!
Just a few suggestions, though -- namely, the biggest problem I had was with getting hit by an enemy. When you land from being hit, you don't have that "temporary invincibility" thing, so it's EXTREMELY easy to be hit again and die before you even regain control. Secondly, the ring-collecting sound effect is totally crapped out. Try to find a better-quality one. Third, the physics feel a little moon-walkish, and the game seems to run at about half the speed it should overall... but I guess there's nothing to be done about that.
Overall, excellent job. I'm just being a bit anal-retentive about things. ;)