I like the idea, but feels like it could use some polish. Enemies occasionally seem to get stuck in an invincible state, the waves start instantly when you kill the last enemy - making it so you're sometimes right in the way of the new spawns, and the bullets are hard to see against the gray background/with the small size initially. Upgraded bullets are great but so slow it almost gets harder to evade certain enemies with the upgrade (like those little guys who chase you).
Design-wise it's solid, difficulty curve seems good too, just a bit too buggy and laggy to be fun all the way.
-cd-