Very fun, once you get the hang of it -- I like how you can get to a "flow" state once the songs become more complex, and that's no easy feat to achieve.
I agree with Quy's idea of increasing the lead time, even on easy while I got 86% (585/676), some sections had unpredictable keys that I could probably only get right by memorizing them.
Some more polishing and variations could make this a full, standalone game (eg even smoother note detection, some kind of fail state, maybe penalties for hitting wrong keys and visual cues too, variety in "maps"?). It's a great concept that can be expanded upon.