Now that you bring it up... I did spend some time searching for hard evidence. I did not come up with anything, but I did see some very pessimistic articles about Groupon. One person did the math that on a $40 for 80 groupon the merchant ends up with less than 25 cents on the dollar! It would be cheaper to just give away 20 dollar store credits, and presumably more effective.
In addition they're not terribly professional when it comes to posting and more importantly, correcting the deals.