The game is amazing, but it's over so fast and with so little replay value that it is far from being worth the price.
After 5 and a half hours, I had in fact completed Brutal Legend. I did many side quests, which are actually extremely pointless as they are the same dumb missions over and over and last around a minute if you're playing slowly. The hassle of leaving the main storyline behind to get and drive 5 minutes for a 45 second side quest is extremely annoying, especially that the only benefit is being able to "pimp your ride" which I hardly used anyways.
The point is, the game is great, the soundtrack is awesome... but it's a rental. The game is much too short to be 70$... I know that I felt ripped off after The Lost and Damned of GTAIV lasted me 6 hours... I couldn't believe how fast Brutal Legend was over.
And once the game is finished... what's left? Considering most of the game is dialogue, I won't be replaying it for a while... and multiplayer is dull and repetitive...
Worth the rent, not the buy.