Not bad. Just needs more depth. It would help to make the words a little bigger too. Definitely needs sound.

Pros-
Interesting game mechanic (Eventhough I have seen it elsewhere)
Love the art style
Simple yet fun
Cons-
No music
No real variation in the stages
Input delay between pressing space and flipping gravity
The game doesn't run particularly smoothly
Conclusion
This could be a great game with a bit more variation and atmosphere and just tweaking of a few things, but it already is simple and fun, so not too much more has to be added to the core concept of the game.