We have been using Restaurant.com certificates for years.
Tip: If using a coupon for a place you have not been before it is always wise to call ahead to make sure the restaurant is still in business. We had several certificates that we could not use because of the places closing up.
Tip: Always check the small print. Many places will not take the coupons on weekend. Only Sun-Thurs. Or they may not take it for specials, fish frys etc. Always check before you go.
Tip: Make sure you tell your server that you have the certificate. There may be restrictions you did not notice on the certificate.
Tip: You get the best choices of coupons at the very beginning of each month. Popular certificates go very quickly and then they are 'out' for the rest of the month. Towards the end of the month you can get bigger discounts on certificates that are still available.
All and all we have enjoyed using Restaurant.com. Many times we have visited a place to eat that we would normally not go, or never heard of but we found the coupon on the website and tried it out.