I really, really liked that game! I wasn't sure at the beggining because there is lots of shooter games and I find them so repetitive... but yours got me hooked. The best feature is probably the COLOR-CODED-ENEMIES! Man, I don't know if you thought about it or if it was natural for you to color-code them, but it sure was helpful to me. The upgrades were great too, I like it when there is lots of them (maybe more would've been sweet... or a special mode when you reach a point in the game where you are invincible or you have a very powerful weapon... that kind of thing would be cool hehe :-P
Anyways, I enjoyed playing and recommend this to everyone reading!
Keep up the good work!