The DDE is going through some changes as of January 1, 2008, mostly having to do with an automatic 18% service change being added to all DDE bills (said service charge is also supposedly going to be added to all WDW dining checks for 6 or more diners). Even with the changes, many still consider it a decent value.
As of now(subject to change, etc.), DDE-cardholders get their 20% discount at the following counter-service places (resorts without full-service restaurants...
All Things POP! (Pop Century Food Court)
Both Port Orleans Food Courts (since PO/FQ doesn't have a Table Service restaurant)
All-Stars...Movies, Sports and Music
It is not accepted at any counter-service places in the parks. It used to be accepted in Animal Kingdom at Tusker House, but not since the advent of The Yak and the Yeti and the pending changeover of Tusker House to a character buffet.
agnes!