Interesting concept. I like it. The art was lacking, and sound would have made it better. You didn't get your first upgrade until wave 11, but then, at wave 20ish, you got 2 new upgrades... it felt like too little, then too much. I think spacing them out would be better.
Also, the bugs were very annoying. I think that's the biggest issue. Waves not going out when you clearly have none on screen, enemy bullets going through your waves, just to name the biggest two. Also, the fact that your bullets also hit your own waves was more annoying than a bug... but maybe it was a bug.