There are some great moments in the earlier episodes for sure, but I believe the filmmakers went a little too far in trying to please everyone. As in any other genre of art or music, there was great punk rock and well... there were shameless posers.
Part four was a complete throwaway. If you understand the ethos of Iggy Pop, The Buzzcocks, The Clash, Sex Pistols and other pioneer punk bands, its pretty hard to reconcile it with Green Day and the like. Post-Nirvana punk took a hard tack towards pop/commercialism. The music suffered as well.
At one point in Part 4, one of the Green Day people actually bragged about how many records the band sold and pointed out that they out sold Nirvana. Oh the irony! These dudes are just not too bright. Its cringeworthy listening to them speak.
Let's put Green Day and their ilk solidly in the shameless poser camp. Its skateboard pop.
Say what you will about Kurt Cobain, but his demise was eerily similar to the death of punk rock in the early 90s. When your art form is based on fierce integrity and honesty, it can't survive in a bought and sold medium. One might as well just kill themselves.
