OK, first impressions now that my fanboy glee has subsided and I'm a good few hours into it.
Good:
Everything visual and auditory. The landscape looks exactly like how you'd want it, the voice acting is fantastic all-around, the soundtrack is awesome, the character models are all simultaneously hilarious and awesome.(Especially the Headbangers)
The writing. It's Tim Schaefer, though, I'd expect no less.
The facial animations are REALLY impressive, the same quality you'd expect from a major animated movie from Pixar or somesuch.
Regular combat isn't quite as elegant and simple as it was in Arkham Asylum, which, IMO, sets the standard for fighting in an action game, but it's close. There's a bunch of semi-extraneous attacks for those who like variety, but I get along pretty well by mashing X and Square.
Car combat. No aiming, just point in the general direction of bad guys and start shooting. Sounds really dumbed-down, yes, but, since it's not the focal point of the game-this isn't Vigalante or Interstate, after all (Though I wish someone would remake those games...), it works really well and, more importantly, is fun.
The Could Be Better:
Driving. Your car bounces and flies around whenever you hit stuff, and, in the racing minigames, your opponent can shove you all over the road, and you can't do the same. Plus, although this may be a PS3 controller-specific complaint, steering is too fidgety and it's too easy to accidentally boost with L3.
Stage Battles. They're really chaotic, and there's not enough control over your guys. It's either 'Everybody Stay!', 'Everybody Follow Me!' or 'Everybody Go And Fuck Stuff Up In That Direction!' You can't split forces, individually control troops, stuff like that. However, since the unit AI is pretty good, you can get along well enough with the odd command and making sure to keep summoning more troops when you need to. If there was a way to rope up a unit or two to follow you, it'd be nice. You can kinda do it by getting your Headbangers to Mosh Pit with you, but that means that *all* the Headbangers will follow, which doesn't work well for the rest of your units.
The Bad:
There's only one Dethklok song in the soundtrack! Lame! I also found it kinda weird that there's no Metallica in the soundtrack, given that they're giant media whores.
Ozzy Osbourne is far easier to understand then usual, and this confuses and frightens me. It's like they got the world's most clear-speaking Ozzy impersonator, instead of Ozzy himself.
