Fun game - very challenging. The music gives a good overall feeling to the game that fits well.
I like the graphics - especially the background. The game worked well as is, but it might also be nice to give the sense that the player is moving, too (scroll the background?). Just an idea, but not critical.
I had a few tight places where I'm not sure I could have survived - my space bar didn't always seem to react as quickly as I would have liked. Maybe you could make sure it is available as soon as possible after the player lands.