We've been "members" or whatever they call it for several years now. It's a lot simpler now that you don't have to have a special Transmedia card. You just use your regular credit cards -- which you have to register with them first. If the restaurant is part of the program, you get a charge on your bill for the full amount on the charge slip, and usually on that same bill (of the next) you get a credit for the % discount. The good thing is that even if you don't know that the restaurant is participating, you still get the discount (kind of like a gift on your cc bill).
The "buts" as far as diners is concerned, though, are that there can be some screwups with which cards you have registered. We basically use only 2 cards (1 Visa, 1 MC) and Amex only when the place takes nothing else. We've registered all of the cards with Idine, but they don't always give the credit on the MC and Amex. Then we have to complain, etc. AND there's an annual fee, which means if we don't get that much in discounts in 1 year, it costs us to participate.
As far as the "but" for restaurateurs -- I once asked an owner who participated, and she said they had a lot of restrictions. Don't remember everything she said, but basically she found the value of the loan she got from them not worth the effort she had to make to "repay" it.
This is all probably a lot more than you bargained for, but I have to live up to the "Bigmouth" title I gave myself.