I initially found the controls to be quite difficult to use and I kept having the notion to press on the keys between F and J. After the second time playing the same level I started to get used to the controls and found the game to be more enjoyable. I don't know if it was intentional but it's quite clever using the 'home keys' (F and J) as most keyboards have notches on those keys.
I would probably start the first level with maybe only four collumns (D,F, and ,J,K) to get new players used to the controls. I noticed I would hesitate a lot when I saw any of the markers at the very top or very bottom since I couldn't quite figure out what the right timing should be since the icons indicated a direction (up or down) but I wasn't sure if it made a difference.
Including the highscores was great for adding re-playability and made me want to try to improve my score.