I really like the concept here; basically keeping your heart beating (did you use RhythmNotifier like I did?)
Aiming was pretty difficult, especially because I didn't know when the gun was about the fire. It also wasn't obvious to me how my beat-matching was affecting the gun's performance. I think there's room to play around with how the gun interacts with the beat-matching. Maybe every five beats you click and hold so you can time the gun firing? Maybe the gun is constantly firing but only when on beat? Cool system, worth tinkering with IMO.
I wasn't able to play very long because of the full screen flashing. I understood that it was to help me know if I was on beat or not, but because I had to play the game full screen I just couldn't handle the screen constantly flashing :(