Sometimes there are minor annoyances here and there. For instance, when the first boss uses his gun, you're locked in cinema mode until the first shot is like a foot from your face. You're likely to use a continue dying from that shot or the one directly behind it alone, which is annoying.
I also experienced a glitch with the second boss. None of his shots went off, but that also meant I couldn't kill him. I was forced to restart.
My point is basically that the game itself is fine, and it's fine if you want to make things a challenge. If you're gonna make the game a challenge however, please make sure there aren't any bugs or points where the player might die simply because of the game and not because the player made a mistake. A challenging game is gonna be frustrating; a challenging game with bugs and points that kind of force a death out of the player is gonna make people stop playing.
Otherwise, like I said, good game. I enjoyed the simplicity of the controls and the decent difficulty. Reminded me of the SNES.