Much of the enjoyment of the game is the music selection and orchestration. I have youtube playthroughs of many of the missions (sadly a lot were hit with claims, but w/e) and in putting them together I'm struck by how appropriate the music choices are for the sequences.
It's not *just* heavy metal in the soundtrack, but it is all essentially rock. And it's *always* appropriate.

The story is well written. Virtually all the stories in Tim Schafer games are excellent. The gameplay can feel repetative because it's not a hardcore brawler or a hardcore RTS or a hardcore pikmin/overlord. It has all of these bits that come together with lots of diverse forms of play. The open world combat spans the gamut from "yeah, I can solo this easy" to "omg run away!" There are small encounter missions with minigames and just out-and-out combat. There's lots of landmarks to find that explain the world and lore.

I daresay you can liken it to the GTA/SR games. They're terrible driving games. They're terrible shooters. They're terrible at everything *except* writing and providing a vast world to explore and screw around in. In sum, though, it provides a fun experience.

If you actually *hate* rock music, it's possible you have a bad time. Not because the game depends on it, but because Brutal Legend also draws on a lot of the same tropes. The landscapes are out of metal covers from the 70s and 80s. The backstory *is* Metal. I think, though, if you're open enough to playing the game at all, you'll be fine with it.

I *love* this game. ffs... I figure out tabs for some of the music from the OST.

I think you'll be able to get over your "not a fan"ness of metal and enjoy the game.

One last thing, if you love Jack Black, you'll be a bit disappointed, as his character is not as Jack Black as some Jack Blacks you might have seen. If you hate Jack Black, you'll be happy to know that his character is not as Jack Black as some of the Jack Blacks you might have seen.