Sound City. Documentary about a famous LA music studio and how the music industry switched from analog to digital. It was pretty fun to see all the famous bands that recorded there. I got tired of Dave Grohl - I get it Dave - you're so retro you need to be interviewed sitting in the driver's seat of an 70's VW van. Gets a B from me.
I'm not sure if I've posted this before but the best music movie I've seen recently is "Searching for Sugar Man". Really interesting documentary about a 1970's American musician who's album became huge in South Africa without him knowing. Gets an A.