1) It's very easy to use, took about 5 minutes to setup.
2) People don't click the ads much, I've had over 20,000 impressions without a single click.
3) The service is very similar to AdWords contextual / placement service.
4) Hands down the coolest feature is the one that allows users to rate a merchant and share with others, no doubt Google will bite this feature soon enough.
Here you can get a glance of my home screen.
Here you can see my actual ad on the #2 resume book on Amazon.