Loved it. This type of game has been done before, but you've clearly looked at them and borrowed the best elements from the lot of them. The controls were super smooth, upgrades made sense, and the bosses had great patterns and increased in difficulty in a sensible manner.
My suggestions: First, add medals; there's no reason not to with this sort of game. Second, carry that comic book theme throughout the game. Give each boss a cool page and a name of some kind before each fight (That can be skipped after the first time you see them.).