I've used tons of them. I've had a couple where the companies went out of business or stopped accepting the coupons, and Restaurant.com replaced the certificate with a generic certificate that I was able to use to get a new certificate for a different restaurant.

Make sure to read the restaurant-specific restrictions closely and research the restaurants. Many of their restaurants aren't that great and often the restrictions make it only a slightly good deal (rather than the great deal that it sounds like). There are some real gems to be found, though.