The Online Publishers Association and Dynamic Logic came out with a study on the effectiveness of online ads (PDF). Seems ad networks are "out" and being an OPA member is "in" - at least if you're concerned with ad awareness metrics.
A key finding of the study was that ads placed through ad networks and portals generated the smallest change for branding metrics, and in some cases made no difference for advertisers. The research also found that advertising effectiveness scores (in raising awareness, creating message association, generating brand favorability and driving purchase intent) on participating content sites (OPA Members) were numerically higher than scores for ads placed on portals or ad networks and the industry as represented by MarketNorms® (a marketing effectiveness database from Dynamic Logic of 4,882+ campaigns across more than a dozen industries collected from over 7 million surveys worldwide).
+ Ad networks were shown to provide advertisers with the smallest change in ad effectiveness - including no change in purchase intent;
+ Online ad awareness metrics were 21 percent greater for ads on content sites than overall MarketNorms and portals, and 50 percent more than ad networks;
+ Purchase intent was about two-thirds higher on content sites than portals, nine times that of ad networks and 50 percent greater than overall MarketNorms;
"Ad network performance has continued to trend downward, with this wave of research showing collectively that ad networks provide advertisers with no increase on purchase intent," said Pam Horan, president of the OPA. "In other words, brands see a flat line on purchase intent if they do not advertise versus if they advertise on advertising networks. Conversely, the data also shows that overall online ad effectiveness as represented by MarketNorms is trending down, while ad effectiveness continues to increase on quality content sites, as represented by the OPA members."