Well, I made it through medium. I never lost a single song, but good god did I suck at "Cowboys From Hell" and "Bark at the Moon". Still, I guess passing with 3 stars is pretty good. Thank god for star power. So far, I've tried one song on hard, and I barely passed. Well, at least I made enough on medium to unlock both characters, all the songs, and all the videos.
This reminds me a lot of Frequency and Amplitude, where I did well at easy and medium, and then hard just became impossible for me. Still great games, though.