I've had mixed experiences. Though I thoroughly read the terms and conditions, my last experience was bad. After emailing and phoning restaurant.com to double-check that this specific restaurant participated in their program, I bought a certificate, only to find out after the meal that the restaurant would not accept the certificate. It turns out that the establishment's owner was in a lawsuit with restaurant.com. Not only was it embarrassing, I ended up spending unexpected extra money.
I believe that Restaurant.com should take down any restaurant with which it is legally wrangling. The consumer ends up being the one hurt. And the restaurant gets in a pickle, too. As far as I'm concerned, since I adhered to all the terms and double-checked with restaurant.com on the restaurant's participation and since they did not remove the restaurant from their site after complaints and a lawsuit began, the management at restaurant.com is at fault…especially if they refuse to reimburse my out-of-pocket meal costs (from what I've heard, that won't happen).
If I were to ever buy a certificate from them again, I'd certainly phone the restaurant beforehand. However, in my location, most of the restaurants appear to be on the low end, so that is not likely to happen. The restaurant in the lawsuit is more upscale.