This is Spinal is one of the most famed movies in rock history. It is a mockumentary of a rock band struggling with the sudden success they achieve and how they blunder their way through the rock scene of the 70s and 80s. It is one of the funniest movies I've ever seen, and contains so many classic lines that it's a daunting task to remember them all.
The movie was great in itself, and it's nice to get it in DVD format, but the addition of commentary takes the movie to a whole new level. The commentary just adds more quotable lines to the movie and makes the best moments of the movie even better.
The seriousness these guys in the band have when they say or do the most ridiculous things are perfectly executed. The plot is used as a device to deliver jokes and the movie never tries to be anything more than an excellent parody of a genre that at the time might have begun to take itself too seriously. Even if you're not a huge metal or music fan, this movie is a must see comedy.