I'm a fan of gaming, know nothing about metal and this game is completely off my radar in terms of purchasing it. As one of the people who likes to throw grenades at aliens I can tell you that if this does come out in the last quarter of this year it will be dwarfed by Modern Warfare 2 and Halo: ODST which many people will get solely because of the Halo Reach multiplayer beta if they're not enticed by playing as someone other than Master Chief. Brutal Legend is a niche game that's going to draw metal heads who feel the game was made for them (because it is) and hardcore gamers that are aware of the developer pedigree the game is coming from. I also think selling a game voiced by Jack Black is tougher than they may have thought because even as someone that kind of knows a little about games I can already see where people would think this is just some cheap novelty game with Jack Black being Jack Blacky.
I know it's probably going to be a very good game in regards to gameplay, but it's hard to want to play it not having any interest in essentially what the game is about. I also live in Florida which doesn't count as this universe so there may be a billion metal people out there waiting for this game the day it comes out, so who knows? But with games like Halo: ODST, Batman Arkham Asylum, Modern Warfare 2, Left for Dead 2, Assassin's Creed 2, and Borderlands all coming out within eight weeks of each other (crap) I don't see a lot of people taking a chance on Brutal Legend that aren't already familiar with that universe... Unless there's a kick-ass demo over XBL, and if that were the case I'd probably be swayed to check it out.
EDIT: The games I listed are games that I specifically want, not that anyone else is looking for them, but I left one off of the October list that I'm dying for and that's Splinter Cell Conviction.