Made it past level 5, which looks like to be the last. I found the upgrading system to be really motivating! Once you get more bullets and faster attacks, it becomes real fun. Maybe those upgrades should be available a bit faster, because at the beginning it is quite challenging and not as fun. The combat is pretty good. If you want to improve how it feels, you could add some juice: Enemies exploding when they die, camera shake when you hit an enemy, stuff like that!
Nevertheless, I enjoyed it. Good job!