I consider myself relatively knowledgeable when it comes to Japanese cinema, still I was completely baffled when I learned Joe Hisaishi (the famous composer) had also directed a film of his own. Quartet turned out to be quite alright, at the same time it's pretty obvious why Hisaishi didn't choose to pursue a career in directing.
Hisaishi really embraces his musical background and delivers a drama that has its roots in the world of music. Four young musicians start a quartet, but they soon find that playing together is more difficult than they originally suspected. By facing their own personal issues, they'll learn how to get better as a group.
Hisaishi spends a bit too much time on the music itself (some of it taken from his former films). The performances are decent, the drama is nice enough, but at times it feels like filler in between the musical bits. It's certainly not a bad film, but the balance is a little off and if you care so much about Hisaishi's music, it's probably better to just catch one of his concerts.