This is a pretty simple and nice game :) It's easy to understand, and quick to play and complete :)
One thing that bothers me is that you can't tell the difference between the immovable walls and the background in the stage where you flip gravity and is invincible on the ground. That's just a nitpick though, and it was quite obvious what the solution was after I died there.
All in all, it's a fun experience, in spite of it's very few levels. You could easily expand this concept to more inputs and outputs, and make many more interesting puzzles if you so wish :)