andrew frank at gartner on google's ad exchange
Worth reading in full; I loved these bits most.
Agency buyers and ad networks, which often find themselves in “co-opetition,” are highly motivated and empowered to put considerable price pressure on additional intermediaries, especially if their name is Google. In its AdSense search business, Google has the advantage of opaque pricing and weak competition; in display exchanges, transparency reigns, and Google has plenty of competition. Agency buyers and ad networks, already feeling squeezed, have in common buying power and experience, which will drive exchange margins to a minimum. On the other side, publishers, especially those who consider themselves “premium,” are feeling even more squeezed, and are talking with agencies about private exchanges for their own networks that could minimize price friction in the value chain. Such developments will drive Google out toward the long tails of sites and advertisers, where display tends to lack the performance qualities of search.
Of course, as Frank points out, "many have lost betting against Google."