Are iAds popular? In 3+ years of owning an iPhone I'm not sure I have ever seen one in the wild. I've used a fair number of free apps that have ads but not a single one of them used iAds.
I think the key difference between iBooks and iAds is that there is at least some demand from users for the former - some people buy iPads to primarily use as eReaders, whereas nobody is buying iPads to look at iAds.