I tried it. But I really didn't have control over how it looked and my internal matching plugin was better so I took it off after the test. I'll give it a go again in a few months as I am sure it will be improved. I imagine that it is a pre-cursor to an Outbrain like product.
I've been using it and it has improved as far as the looks go. I use the Response size. Today I noticed the matched content has been replaced with a responsive ad. Not sure if that means they're having problems with showing matched content or maybe they're updating it?
If I was approved for it, I'd definitely try it out. I've been using Taboola and the CTR on its ads are generally much higher than they are for the regular AdSense ads on my site. So users clearly are attracted to them, and if Google can figure out a way to make them work -- and honestly I'm surprised they haven't introduced a similar ad format for more AS publishers before now, considering that Taboola and Outbrain, another provider like it, have been around for a few years -- then they'll take off.