All the replies are correct, just want to add one thing. Tell the server about the coupon soon as you sit down. Make sure they are still accepting the coupon (if they don't, Restaurant.com will refund), ask if there are any limitations. And some servers may be unfamiliar with the coupon and will have to speak to another employee. Sometimes you cannot use the coupon with a special offer, like a BOGO offer for example.
Better safe than sorry.
I have been using them for no less than 6 years, very few problems, one place closed before I could use my coupon (maybe a good thing), one place stopped accepting them when they became very popular without the offer.
But, most times we have a wonderful meal at a nicely discounted price.
Oh, one more thing, never pay full price for any coupon unless it's so popular that it sells out at the beginning of the month, before any discounts are available! Wait a few days and the discounts start to show up, they usually max out at 70%, and a few times a year 80%. When you get an 80% offer, use it.