Jeff Ferguson: Increases in the client`s true KPIs via the organic search channel. By "true KPIs" I mean how the client makes money (leads, revenue, etc.), not vanity metrics like traffic (unless they make money from traffic, like with advertising-driven sites), rank, and so on.
Peter Hatherley: One that broadens your reach in long tail search
Perry Bernard: One that delivers leads or sales proven through gains in GA source / medium = google / organic or by phone conversion.
How would you define a successful SEO campaign? 
