This is really nicely done. I think there are a few implementation issues, but I think a few tweaks would make this stupidly addictive!
But dang, one thing I will say, as a musician and someone who likes absurd stuff like StepMania :-P, this is a really creative way to take those concepts and make actual brain training fun! It's quite difficult, especially when the circle on one side climbs up before the other side xD.
The tutorial is very good, but I would suggest indicating both hands. Just the order I did things in, I read the controls, saw I needed a right and left trigger, and was confused why only left was doing anything at first LOL--small potatoes here, though.
For any of the slightly faster-paced stuff, it gets a bit jumbled quickly.
The biggest thing here, I think, is that the feedback (the green flash on the wheel) is when the square crosses it, not when the user hits the button. The only visual cue the player gets is the rating which isn't precise.
I think if you kind of made that indicator a "shadow" of the square (and bigger) - taking a page from DDR/StepMania again here, and the aim being to hit it when it visually lines up, and the "flash" or whatever is the user's input, then it would be a lot clearer. Additionally, I think if the circles were larger, that would make the squares more spaced out--which would let us actually "see" the rhythm differences better. Stuff that lines up and offsets like 5 on 7 or something is really tough to pick up on with the current....scale ;).
Making that feedback correspond with the input would also make it easier to pick up on if there is, indeed, lag. Calibration based on a super slow/obvious tutorial level could address that, but I know it's not like we got all year to make these :).
Anyhoo, just getting detailed there because I really like it, enough that I might clone this sucker and take a peek if I have the time!