I really enjoyed this game, it is very unique. I like how you mapped out the beats to the key presses so the player can play long with the background song. ( I also like heavier music so that helped ;) ). The backgrounds were appealing and were well contrasted to the big notes, making them stand out.
The only thing I'd suggest adding would be a scoring system based on how accurate you were with hitting the notes, such as "perfect=10" - "good=5" - "ok=2" etc. I think that would make it a lot more challenging. Other than that though this is one of the games that stood out to me and played a few times. Nice work :))