The game has a great feel to it. I'm sure I have palyed something similar before, but who am I to complain with my game :)
If I had one think to point out it would be that this feels more like a level 3 (or 5, or 10). What I mean is that an introduction level with less circles and enemies would help the player get into it. And then at a certain score you could move to the next level and so on.