I could give you more case studies than I can shake a stick at. Ditto the guys at SEOmoz, Distilled, Bronco, Ayima, SEO Gadget, Verve, Outspoken Media... Discl

I could give you more case studies than I can shake a stick at. Ditto the guys at SEOmoz, Distilled, Bronco, Ayima, SEO Gadget, Verve, Outspoken Media... Disclosure: I know most of the guys at those companies. That aside, their work and expertise speaks for itself.