The best parts of Tim Schafer games rarely have anything to do with the gameplay. If you approach Brütal Legend thinking "boy, I bet this open world action game with RTS set pieces is going to be a blast of fun!", then you're probably going to be disappointed. That's not to say that Legend is boring, but the action game parts are average, the RTS set pieces are simply functional, and the driving and open world aspects are merely present.
The great part of Legend is in the funny writing and badass situations and designs. Main character Eddie Riggs (Jack Black) can do some pretty awesome things, like shoot lightning out of his guitar and literally melt faces with a wykkyd guitar solo. He also has clever one-liners and is surrounded by people who do as well. Not every gag is a winner, but I didn't think they fell flat very often except through repetition.
The images in the world vary from good to great, not just through rendering power, but in design as well. Most of it is lifted wholly from metal imagery, so they can't really take credit for all of it, but the original work is strong.
Of course, if the game shines in the writing, scenarios, and design, then why put it into a medium that needs more than just those three things? A pretty good movie or TV series could fall out of a well-done trio of these elements, but they don't need the element that Legend is lacking: a compelling reason to interact.
There are plenty of WAYS to interact, whether they be the aforementioned action sequences (beating guys with your axe (weapon) and blowing them away with your axe (guitar)), driving sequences, RTS sections, and a scattering of other things. However, I can't imagine doing any of those things more than once if there weren't the promise of more of the good parts to follow.
I think Schafer games have always been this way, but it was a bit more difficult to notice when he was doing graphical adventures. Those games are almost entirely interchangeable except for the writing, design, and scenarios, so even though that's all he ever did right, that's all ANYONE ever did right in that genre.
I don't blame him for leaving adventure games after they died, but I suspect that his decade since then could have been better spent working in a different medium. Make Brütal Legend non-interactive and it would be somewhere between Metalocalypse and Tenacious D, but probably better than both. Leave it as a game, and it's pretty ok, but absolutely worth playing if it's the only way you can see the non-interactive parts.
Given the existence of the internet, I doubt that it is.