I've subscribed to their service about a month ago. Thus far I have received 433 hits that were referred by people clicking on my ad. That's about .32% of the total number of hits on my website during that month. It's not significant, but it's free. You'll note that one paid plan assures 1,500 hits per month (which would amount to about 1% of my website traffic). I'm not sure it's worth the cost... yet.
My ad appears in three different size options on subscriber websites, and my website traffic reflects the number of hits generated by each one.
I'm going to let this go a few more months before I do anything else with it. I'm already buried in deadlines and grandchildren. I'll keep you informed.