I like the idea. The music in the main menu is also very nice. But as I'm not into rhythm games, it's a bit too hard for me (as you already stated in "known issues"). After a few tries I managed to progress a few stages in easy mode. I'd really like to see a quick "Try Again" button which quickly restarts the run with the previously selected difficulty level without showing the tutorial.
I also found a few other small issues:
- You can actually always press all 4 Buttons at the same time since there's no penalty to hitting notes when you shouldn't.
- Missing "Quit game" button in main menu (Alt F4 needed)
- Inverted scrolling in the composition view feels weird