Good and fun music game built up the same way as Guitar Hero just with a keyboard. You can change the difficulty from decent to impossible. It's always a great feature in music games that you can add your own music - in this game you even get an achievement for adding 10,000 songs(!).
One little note: This is possibly the only game ever that gives me nausea. After one song my vision is all weird, and after 4-6 songs I just have to stop.
It's just a bit sad I will never get all achievements in this game. I don't think I have 10,000 songs to add, and the hardest difficulty is just impossible for me.