There are listings that show which restaurants take the DDE. There's a very inclusive list on All Ears that compares discounts from DDE, AP, and
DVC.
Here's what my experience has been. Most table service restaurants in the parks accept this. One exception I found was Teppanyaki (Japan table service).
The only counter service I can think of in a park were in AK. They take it at Tusker House and Flame Tree. Someone mentioned pizzafari earlier. This makes sense because other than RFC, there's no sit down restaurant in AK.
Many of the table service resort restaurants accept it. And those resorts that don't have TS take it at their food court.
One thing to consider is that the AP will get you a 10% discount at many of the restaurants that give you the 20% DDE, so the actual savings is only 10%. But DDE gives you 20% off drinks as well as food (including alcohol).
I ended up buying this just before my spring break trip, and it paid for itself on that trip alone. I did use it a lot at the AS Sp food court (usually saved about $5 each time). I also used it at: Boma, Sci Fi Cafe, 50s Prime Time, Trails End, Kona, Tusker House and Whispering Canyon. My DDE savings were around $90, and that was feeding 3-4 light eaters. But I would have had the 10% AP discount at several of those restaurants, so my true DDE savings was closer to the $60 cost.
And my final comment is that the $60 price is a discounted price. That's what I paid. I can't remember if it was because I was a FL resident, but that's what I think it was. I think the general price is $85.