Music Hero is, mechanically speaking, exactly like the phone versions of Guitar Hero. You've got three bars of notes coming at you and you've got to press the notes when they get to the bottom of the screen. The game comes pre-loaded with three songs, all of which are instrumental and pretty much awful. The game has two difficulty levels, which manage to range from really easy to downright impossible. Or maybe my fingers are too slow for the higher difficulty.
What sets Music Hero apart, allegedly, is that you can play it with songs you have on your phone. And, amazingly, this almost works. It will try to come up with something that sort of goes in time with the song you selected, but the tech is too limited to pull it off convincingly. If you try to do something weird with it, like play the theme song from Halo, it will completely lose its mind.
If somebody were to come up with a way for the computer to automatically generate a legit rhythm game note set on the fly, it wouldn’t be free. Perhaps Music Hero 2 will be better?
For now, though, you're better off sticking with the phone versions of Guitar Hero, even though you'll have to throw down some dollars for those.