I think for some types of sites it will be very effective. For example, if you have a game site you may have noticed that most of the time, adsense carries ads for other game sites... But your visitor already is playing game through your pages and is not likely to jump away through adsense to another game site. You do get clicks, of course, but probably a game sites click ratio is below average. But if ads start appearing that relate to the visitor's other interests, what will happen then? Perhaps when your visitor is finished playing games for the moment, and then notices an adsense ad about something of personal interest (not games) - like autism or shoes for sale, etc., that visitor maybe more likely to click a link and your click average should go up. My guess is that Google will show a mixture of content specific ads and visitor related ads that will be fine honed and produce more ad click revenue for Google and the website publisher... And for the advertiser as well.