There has been a fair bit of trash talking recently regarding iAd. [Daring Fireball - Build & Analyze - Bloomberg] Much of it centered around whether iAd is a big flop for Apple and a bad deal for developers.
I have been running iAds since day one and in my experience it isn’t that iAd is doing poorly, so much as that mobile advertising is a horrible market.
iAd is actually the best network in terms of revenue, crushing the competition. It is just a market in which making money at all is a struggle.
Some data from my Audiobooks app in June to back this up:
iAd:
Impressions: 994,711
eCPM: $2.20
Revenue: $2,183.51
Google Adsense (typically the second best):
Impressions: 474,167
eCPM: $0.98
Revenue: $464.82
All remaining unfilled ad requests go to an aggregation service which bundles all the smaller networks, these average out at an eCPM of $0.42.
iAd certainly could improve a bit in terms of fill rate. Though even that has been steadily better since launch. Now hovering around 60%. 
In summary, they are still the best network out there, outperforming the nearest peers by around a 2X margin. You can still make pretty reasonable money from advertising, especially as a way to capitalize the low end of your customer base.
The real money of course still in Paid apps and In-App Purchases. During the same period this app suite made about 3X the money from purchases than it did from advertising.