Eh, just sort of mediocre. I voted 4 because 3 would be too low because I played it all the way through, but just sort of uninspired.
A couple of minor gripes: it wasn't really that hard, the upgrade screen didn't come up automatically after finishing each round, the repetition of the mid--round bosses was annoying, the tetris homage was kinda funny but wasn't a good enemy at all, I wish that I knew that double-jump would be unlocked, the final boss was not challanging, the laser enemies were unavoidable, so on and so forth.
Not a bad effort, but a really good complete game.