Hi, I like your game a lot. :D I keep coming back to play it because...its fun. :) I love it when games are simplistic and easy to understand and I also just like top down shooters. The upgrades add a lot of fun to the game and the overall gameplay mechanics work really good.
There are two things that are a little bit probelmatic for me:
1. The movement is a bit too slow for my taste. But it's just a personal preference. I still think it works fine.
2. This is a little bit frustrating: Enemies keep spawing in me and than I die and than it's over. xD Would be cool if that wouldn't happen.
But congrats on your game. I like it. :)