I've used many restaurant.com gift certs. On a couple of occasions, the restaurant refused to accept the coupon (both timed due to a dispute between the restaurant and restaurant.com). All it took was one email to restaurant.com customer support and they gave me enough credit to purchase another gift cert of equal value.
From reading the comments on this site, it does seem that they do seem to sell a load of goods to the restaurants. But, they do seem to take care of the consumers. Now, whenever I plan to use a restaurant.com gift cert, I call the restaurant BEFORE I leave home to ask if they still accept restaurant.com gift certs. That's alot better than having a very embarrassing situation at the END of a meal.
But, I have noticed that there is clearly resentment in the eyes of the restaurant managers and waiters whenever I pull out my restaurant.com gift cert. It looks like many restaurants sign up for the program and then regret it later only to discover that hundreds of certs have been sold already that must be honored.