A lot to like here. Loads of charm and character, cool concept, nice visuals (especially the bullet effects) and good music.
I did find that because the levels are fairly large scrolling areas and I don't need to aim to attack the boss, all my attention was on the bullets and the rhythm window; this meant it started to feel a bit like I wasn't really "fighting" the boss so much as playing a rhythm game while dodging bullets. Not exactly a criticism, just something I noticed. It would be great if the rhythm game part of it was a little more varied and integrated into the music of the game too.
Again, what's here is really cool and impressively done though!