The enemy shots felt like they fired a bit too fast compared to how fast I could move, and everything else could be tighten/sped up a bit more (game over screen most noteably), but overall the concept and the execution is great!
I think if you made a full version, weapon/music switching could be a fun addition and +1 to Ikkir's suggestion of enemies shooting during offbeats, it'd make it feel a lot more rhythmic.