I'm pretty impressed with this - I purposely fed it a song I thought would kill it ("Fuzz Universe" By Paul Gilbert) - It did an impressive job of capturing many of the underlying chords, while ignoring the lead lines over the top. I notice that it is not really great at capturing very fast chord changes, an has some trouble with varying time signatures, but great first effort. It would be pretty cool if you could upload your own MP3 to it, and get a result back - that way you could generate the output off a recording of yourself to distribute to bandmates.

Edit: Later, that song did kill it, as the changes got faster/harder.

Also, it doesn't seem to have a complete set of possible chords - one song to check would be "A hard Day's Night" by the Beatles. It has a difficult and distinct first Chord which might be valuable to test against.