Tried the demo after my friends saw the Kickstarter and recommended it to me because I've played Muse Dash before. (And this game is like Muse Dash, but coming at you from both directions!)
I mostly played on controller for my time with this demo and thought the button layout works well. I had a harder time with the keyboard controls (so echoing others on allowing button remapping).
I thought the controller rumbling in time to the music was a fun touch. It also helped me on the slower songs when sightreading.
Could the team add some kind of feedback when a note is missed during play? I find myself getting to the end of a song and noticing I've missed more notes than I realized. Having some kind of audio to indicate that I've missed a note would help me gauge which parts of a song I need to practice.
I also had issues differentiating notes and the background sometimes, but saw your comment about letting users customize colors and dim the background; both of those sound like great options!