We use the certificates alot. Yes there are some restrictions, but we can live with them, for a chance to try a new place, without betting the farm on it.
We try to purchase the certs when they are on sale. For example, a $25.00 off cert, normally carries a price of $10.00. During 60% off sale, the cert is $4.00. Go to a place with a $40.00 mimimum.
$40.00 less $25 is $15.00. add the four dollar price of the cert, and you are able to eat $40 worth of food for $19. And since we like to try new places during the week, rather than a busy Friday or Saturday night, the restriction works for us.