Disappointing. Japan has a pretty sketchy track record when it comes to films about bands and pop/rock music, On-Gaku seems all to content to continue that trend. Apart from the differentiating art style and the dry comedy, it's really just a rather bland and by-the-numbers film with a horrible soundtrack.
A group of high school delinquents is so bored, they decide to form a band. While rivaling gangs are trying to settle scores with them, the three boys get some instruments and without any formal training try to find a sound of their own. When they score a gig at a local festival, it becomes a race to perform.
The crude and simplistic art style is what jumps out right away. It isn't very pretty to look at and the animation itself is quite poor, but there are some nice style variations throughout the film. The soundtrack is crummy, the comedy not at all that funny and the plot is of the cookie cutter variety. Many people seem to like this film, not sure what the deal with that is really. It's all very "been there, done that".