Cool
You did a great job making this interesting and much more "3D" than many intrument sims you see on Newgrounds. Adding beats and a metronome was an awesome idea.
The one drawback is that it is very difficult to make any interesting music using the mouse! Try making the keyboard keys controlled by the keyboard on your PC, that way it would be possible to actually play the keyboard.
It looks like you spent some time making this, though. Nice job.