Very nice experience !
The difficulty curve felt just right and the levels offered enough variety to keep it interesting.
I second the comment about keeping the music going between restarts. I don't think I ever heard the full track more than once or twice, which is a pity considering its quality.
Incidentally, I noticed an issue on a couple of levels where the center of some gravity fields would seem to drift away from their respective planets after a while. Do they, perhaps, have their own orbits rather than being children to the planets ?