Nah, Metallica were once pretty good.
Slipknot =/= Metal, but I will admit that it's not by much. If they were really obscure, popular opinion would probably lump them as metal. They are definitely the most metal nu-metal band, mainly seemingly by Joey Jordison's influence, though what I've heard of their latest album is whiny, trend-whore faux-emo crap.
Man, as fucking annoying as it was at the time, when you look back, wasn't nu-metal such a better trend than emo?