Eh...
I managed to get the boss' life all the way down a couple times and yet didn't see him die. I had about 5 lives left and spend all of them shooting the boss to no avail.
It's unfortunate really. Good sounds, good music, average control/gameplay.
I feel like some of the 'hit targets' weren't quite fair... like a mine or laser would kill me where the animation showed that it probably shouldn't have.
I don't think the game is necessarily difficult, but I found it frustrating that I wasn't able to beat it because of a bug or something.