Most (but not all) of the restaurants that I've seen featured on there seem to be ones that get horrible reviews on sites like Yelp. You also need to watch out for the ones that look like a good deal, but ultimately aren't because they require a minimum food purchase (a lot of them require $35) and also slap a mandatory pre-discount 18% gratuity onto your tab, even if the service is subpar at best.

There are also reports I've seen on other websites that some of the restaurants on there actually give different (i.e. higher-priced) menus to people using restaurant.com certificates. Not sure if there's any merit to that or not, but figured I'd throw it out there for you to know.