Unfortunately, restaurants.com certificates are of limited value unless you know ahead of time what the terms and conditions are. Generally, there's lots of qualifiers: $X dollar minimum purchase, X number of guests, X number of entres, Dinner only, etc. In the past I've been able to get some good deals with them, but the number of restaurants offering a low minimum purchase and low number of guests (for me, 2 is the optimal number, otherwise my conscripted other guests usually feel entitled to a cut of my savings) is pretty slim in my city. There used to be at least a couple of ok restaurants (not fine dining, but good for a *cheap* night out).

Generally, I would say that they're a pretty poor deal. Hope you find a place you can use them!