A good idea, but a few problems
Like most of the other reviewers, I really liked the gameplay and graphics. However, there were a few things that bugged me.
Firstly, there were no instructions beyond the explanation of the control scheme. While moving and shooting were pretty straightforward, I still have no idea how the weapon system works: I seemed to switch between the single shot and triple shot (and cleaning ability) at random. Also, the tier system is now addressed in your description, but isn't explained in the game.
The second might just be an issue of having the quality settings too high for my computer, but several times during the game, the controls would freeze for a few seconds at a time (ie. moving in one direction, shooting in one direction even when I'm not pushing any keys).
It seems like it's easy to add some instructions at least, which I think would improve an overall very good game.