I've used them, and I think they're a mixed bag:
Many of the coupons have significant restrictions, so you should read carefully before buying: Good only for dinner, good only Monday-Thursday, good only on a check of X amount or more, good only on the purchase of 2+ adult entrees. It varies. Just be sure you know what you're getting.
We don't have many participating restaurants in our area -- and most that do participate are inconvenient for us. We have tried to use them while traveling, which isn't always great: More than once we've ended up buying coupons ahead of time and then NOT using them for one reason or another. Once the restaurant was out of business, but most often we just ended up going to other places that looked better once we were actually in whatever city. When we haven't used the certificates, we've tried to give them away (i.e., to hotel staff).
And as others have said, these go on sale CONSTANTLY, so you should never, ever buy until they're 80% off.
We have not saved big with these coupons. We've not used more than half of what we've bought; if more was available right here in our area, it'd better -- we could buy as we're walking out the door.