I use AdMob as my ad provider in all my apps. Recently I've been experimenting with In-House ads. That means I place ads of my own applications in my own applications. Because of AdMob's wonderful web console I am able to track the CTR(click through rate) of my ads. Basically its a percentage of total ads showed and the ads that got clicked.
Now over the past several months I've had a CTR of about 1% without using In-house ads. In house ads have had a CTR of between 3-8%, which is amazing!
On the other hand it just emphasizes, how much rubbish there is in AdMob inventory. Stuff that no one would ever want to click.