I've never thought of Iommi as a paticularly speedy player, but he's no slouch either. I think his real strength is riff writing. Just think of "Children of the Grave" and you know what I'm talking about.
I think when it comes down to it, Black Sabbath was (is) just an awesome band. Geezer and Bill Ward made a killer rythm section, and Geezer wrote some great lyrics. AND... Like him or not, Ozzy's pretty damn good at being Ozzy.