I've seen Mastodon live, and I was pretty unimpressed. Though, it was on the unholy alliance tour with Slayer, Slipknot and Hatebreed, so I suppose they automatically get to be second best band of the evening...
You can kinda tell by the company they keep. Too much of a generic modern hardcore vibe coming through. Maybe I need to give them another listen, but I can't remember thinking them anything special next to, say, EyeHateGod, Cult of Luna, Crowbar, that sort of thing. And apart from the really, really stoned shit (Electric Wizard, Sleep) all that's not really my thing.
Besides which, I'm not sure there's much space left in this world for awesome , heavy albums that rely on massive drug consumption. I mean, they've been coming out pretty constantly since the seventies now, and it seems to have just become a sort of hipster thing almost. Sludge and drone bands, and to a lesser extent American depressive black stuff, is getting WAY more mainstream critical attention than any other genre of metal right now, and it's really starting to bud off from the rest of us in a lot of ways, if it hasn't already. I know quite a few sludgers who'd punch you in the cock if you accused them of being metal in any way.