ElectricWiz
Steal Your Face
Also, one movie everyone on this board must see is American Movie. It's the Spinal Tap of filmmaking. It's also one of the most quotable movies outside of Monty Python and Austin Powers.
Plus, they talk quite a bit about metal, and "Fight Fire With Fire" and Ozzy's "Dee" plays throughout much of the movie.
It's one of the greatest documentaries ever made IMO. When I first saw it, I thought it was a mockumentary beause Mike and Mark seem so much like characters you can't imagine this is how they are ALL THE TIME.
Anyways, go see it.
American Movie is one of my favorite movies. I've seen it many times. It is as funny as you say, but they are charming somehow too, Mike and Mark, and there's just something special about that movie. For other documentaries I also suggest:
Brother's Keeper (about a, 'hillbilly,' murder mystery in NY state)
Paradise Lost (Satanic panic around the murders of some young children - Metallica let them use their music for the soundtrack, but that is far from the reason it is a good movie - though it certainly adds to the atmosphere)
Salesman (about bible salesmen in the 50's)
Capturing the Friedman's (strange pedophile case)