None yet, but just implemented the link yesterday. Will update on any news I have.
This is really interesting to me because, way back when I first started with AdSense, every ad featured an "Advertise on this site" link. It was a head-scratcher for me when they took it away. So it's nice to see it back.
I queried a couple of potential advertisers (two startups directly related to my content and my demographic) a few months ago, and both balked at the suggestion. I pointed out to them that they could get a refund at any time and didn't need to commit for more than a day, but they responded that they'd only participate in an affiliate, pay-per-conversion arrangement. Can't blame them -- free advertising.