DDE is available to either Florida residents or AP holders. If, like me, you are from out-of-state, you need to have at least your AP voucher in hand when you order the DDE. I don't know the charge to Florida residents, but the AP charge is $60 for the card holder and there is an additional $25 for a card for your spouse/domestic partner.
The card gives you a 20% discount on food and beverages, including alcohol, at most table service locations. In resorts without a table service restaurant, you can get a discount on meals purchased at the food court. We did this when grabbing breakfast at POFQ while we were staying at OKW.
The card is valid approximately 1 year from purchase. Typically, if you call on or later than the 8th of the month, the month of expiration will be the following month. For example, a DDE order on January 3, 2007 will get you a card with a January, 2008 expiration. An order of January 10, 2007 will get you a card with a February, 2008 expiration.
We used the DDE during our December trip and it more than paid for itself.