Cool concept! I liked it a lot, but had a few issues:
* Since you can't rotate the camera (I think?), the parts in the beginning where you had to go behind the tall tower were very awkward.
* It was really hard to tell which side of my cube was the side the bullet would come out of, and even it I did lining it up was really hard. I think shooting in the direction of the mouse would have been better.
* If you die, it respawns you at the checkpoint with 10 shots, but when you reach a checkpoint it doesn't give you any if you have less, so you have to kill yourself.
Those are just some minor nitpicks though, I think with some polish this could be a really fun game!