iAd has not been a big hit for Apple. Although the service was launched with a lot of fanfare about ads that you truly want to play around with and unheard-of levels of engagement, iAds hasn’t really taken off.
Part of the issue was Apple’s strategy: they focused on targeting large companies and demanded they make huge minimum buys-in. There was no dipping a toe in iAd: you either didn’t use it at all, or plunged right into your neck for hundreds of thousands of dollars.
Apple’s waffled on that before, allowing ad developers to start hocking their own apps on the iAd network starting in 2010. Today, however, Apple has added an iAd Workbench, making it even easier (and way, way cheaper) for developers to advertise with the network.