Very cool and polished. I like games with upgrades, although I wish there would be more to gameplay than quickly spinning my mouse around the center starting at level 7 - some enemies are very fast, so you need to shoot them ahead.
Sound started to break with some number of bullets on screen. It restored after clearing each level and re-broke once I started shooting in the next one.
I made it to wave 21, at which point I started shooting newer shinier projectiles (upgrade?) and the whole thing started to lag immensely and I got killed.