My fill-rate on iAds in the past six months has been over 50%. In the past month it's over 60%.
I have tried various ad networks. iAds has consistently beat them all by a factor of 10 in terms of revenue to developer. So using iAds results in less ads for consumers and more revenue to developers in my experience.
I currently only use one competing fall-back network (they promised to beat iAds revenue and they failed miserably so will be removed shortly).