I don't think System Of A Down Or Slayer suck. I like them both. Yes, I know there are bands that are far more technical and talented than either Slayer or S.O.A.D., but I wouldn't say either completely suck. I love the emotion and agression of Slayer and System Of A Down because they're different, weird, and odd. As far as real guilty pleasures I believe it would have to be some of the 80's "hair metal". Don't flame me to bad but here goes:
BABYLON A.D., SHARK ISLAND, LEATHERWOLF, LILLIAN AXE,
(There's probably more, but I can't think right know. Also, could some of these go in the obscure band thread?)