Getting to play the Super Mario Theme with a wiimote air guitar is pretty much worth renting right there...but seriously.
My wife is a piano teacher, so of course, she wanted to look into Wii Music. She said that the mini games are actually quite advanced at teaching music theory for people who want to play real music. The "game" itself she said was stupid since it is more just move the wiimote in rhytum, but the mini games are enough for her to encourage all of her students to pick up the game (for ages 4 to 20+) just to learn the musical theory that it teaches.
So apparently it is better then it looks while also sucking.