Using the code "FUN" at checkout gets you 60% off the discounted price of the certificates. Example, a $10 certificate for one of the restaurants I like to go to sells for $4. With the promo code applied it comes down to $1.60. Not their biggest discount but still a pretty good deal. If you're patient you may want to hold out for one of their 80% or even 90% discounts. There is a page where the latest promo seems to always be listed, check the comments for details.
In my area there are a lot of smaller, family run restaurants in the offerings. A nice break from the nationwide chains that frequently offer online promos and coupons.
Do be sure to pay attention to each certificate's fine print, this will differ from restaurant to restaurant and even different value certificates for the same restaurant. ($10 cert may be for lunch but a $25 cert for dinner, etc.) To date I've used a dozen or so certificates from restaurant.com and have yet to encounter any problem