Interesting idea, especially using rhythm to curb spamming.
I think the rhythm aspect was a little bit frustrating since it was purely visual... I think a metronome or some sort of back beat might have helped a lot.
Even knowing the keys, I think it's tough for most to remember notes belong in which key based on text alone. Maybe it's good theory training though? From a layman gameplay perspective I would maybe invert or otherwise mark which notes are in the key you're in... or maybe even if the keys were laid out resembling a piano?
Aside from the game being technical though I could totally see playing this on a Nokia, solid entry!