The Online Publishers Association produced a study suggesting what many of us have known for a long time: that advertising on branded sites consistently outperforms advertising on portals and ad networks. See below for the press release as well as AdAge and MediaWeek analysis.
From the press release:

Across a wide range of advertising metrics, branded content sites outscored Internet industry norms for the Internet 41 out of 43 times according to a new research report released today by the Online Publishers Association (OPA). Additionally, “beyond-the-banner” forms of online advertising such as video, sponsorships and rich media also outpaced industry norms when placed on branded content sites.

From AdAge:

Like nature vs. nurture, the debate between targeting content or audiences is something of a philosophic argument in the marketing world. Some people believe targeting audiences is what matters most — regardless of where they are browsing online; others believe that the quality and context of the website on which a consumer is targeted matters. Now, new findings from the Online Publishers Association suggest that content is king: Ads on branded-content websites are more effective than non-branded sites and outpace industry norms in nearly every category.

From MediaWeek:

At a time of growing concern that the rise of ad networks is commoditizing branded content, the Online Publishers Association has released a report showing that ads perform better when they’re placed on branded content sites.