I really enjoyed playing this game; it was right up my alley. The controls and the goal were easy to understand, the enemies were engaging, and the mechanic of looping around the planetoid was fun. I also really enjoyed the music, because it fit the at times hectic gameplay very well, although I do wish there was fanfare when completing levels. The final boss was also neat.
What I do think could have been better was the art. I've seen a lot of cool things done with these limited shapes that I just didn't see here, and not just because you only used circles. I also think you should have given the 'enemies left' and 'health' counters a visual representation, like a progress bar made out of circles for the 'enemies left' counter. In addition, some simple flashes to show enemies take damage would go a long way.
All in all, great gameplay. Very enjoyable.