In this 2006 musical comedy about two would-be rockers (Jack Black and Kyle Gass) who form a band, it's pretty telling that the best music comes from a short clip of The Who's "Won't Get Fooled Again" and not anything these two perform (they are a real band). It's probably more revealing how the one genuine laugh is courtesy of Ronnie James Dio.
To escape his religious zealot father (Meat Loaf) and heed a vision from his idol Dio, J.B. (Black) moves to Hollywood with his guitar and dreams of rock n roll stardom where he meets K.G. (Gass), another guitarist he thinks is a big star. K.G. takes J.B. (these initials are annoying, but it's how they refer to each other) under his wing, but when J.B. finds out K.G. lives off his parents, he also discovers they're about to be evicted. They enter an open mic talent show for a cash prize, but to win, they believe they need the Pick of Destiny, a mysterious, legendary pick believed to have been carved from a tooth of the devil.
The one-joke premise is how the movie treats these two as if they are forming the greatest band in history. But the music isn't that great, and the movie isn't really funny. Their guitar playing is actually pretty decent, but I didn't find it to be anything special. And the humor is by-the-numbers Jack Black stuff: falling down, swearing, farting, and generally just mugging. That's been funny elsewhere, but unlike Tropic Thunder, in which he had Ben Stiller, Robert Downey Jr., and others to play off of, he doesn't really interact with anyone outside of Gass, who doesn't really do much. Most of Black's antics are for the audience; he feels less like a character and more like someone acting out in front of a camera.
The first five minutes set the tone for how the movie could have gone. After his father spanks him and tears down all his music posters, young J.B. prays to the poster of Dio left hanging. Amazingly, Dio comes to life and sings "In the city of fallen of angels, where the ocean meets the sand, you will form a strong alliance and the world's most awesome band." I don't know who wrote those lyrics, but it does sound like something Dio might sing. Had the whole movie kept this mock epic tone, it might have been more memorable. Sure, there are the medieval cards used for transition, and the duo battles Satan (Dave Grohl, of all people) at the end, but those feel tacked on.
I guess my biggest complaint is the movie seems to profess a love for hard rock and heavy metal but offers no insight into its appeal or do anything with it. It's a very superficial examination of the genre. I'm not saying it had to reverent of metal. This is Spinal Tap is a hilarious heavy metal satire because it gets all the details right. It played off the genre and its musicians' foibles. Jack Black is doing his shtick, and he doesn't need any music for that.
There's was one moment I wanted to turn off the film because I felt insulted the movie expects me to find it funny. Early on, we see K.G. instruct J.B. in an exercise called "cock pushups." When sneaking around the Rock and Roll History Museum, J.B. becomes immobilized by security lasers and can't press the off button. Thankfully (for him anyway), he remembers his earlier training and presses it with an erection. I would have shut off the movie, but I was watching with friends who love it more than me (my friends and I need to start selecting better movies).
Roger Ebert has said the difference between good and bad raunchy humor is the funny movies use bodily functions and crass material to build jokes while the unfunny ones just assume those acts by themselves are funny. Tenacious D falls into the latter category. My advice: for music fans, listen to albums by any of the bands mentioned in the movie: the Who, Black Sabbath, AC/DC, Dio, Queen, Van Halen. For Jack Black fans, I'd suggest Tropic Thunder, but a movie shouldn't be reminding you of the other things you could be watching or listening to.