I'm an unapologetic Metal Head (Have nearly every Iron Maiden CD! Love Dio, Black Sabbath, Judas Priest, Malmsteen, Helloween, etc.), and have to say that Tenacious D captures the heart of Heavy Metal in short, succinct masterpieces. I know the CD is a "soundtrack" but the music could definitely be incorporated into a Rock Opera on the order of The Rocky Horror Picture Show. The movie wasn't great, but the music more than makes up for it, no matter how short some of the songs are.

Especially cool:
Kickapoo -- Killer song. The riffs are driving, even the acoustical ones!
Classico -- Sweet little 1 minute song comprised of Bach and Mozart tunes. It's good to see honors paid to the only form of music that's better and rocks the hardest: Baroque/Classical.
Beelzeboss -- Worth the price of the album, IMHO. It has all the components for a mini-opera.

I grant that there's a LOT of edgy and offensive language, but if you can handle the F bombs then the experience will pay off in spades.

Musically satisfying. Get it!