In some ways, yeah, 20% isn't all that much, it probably won't break the bank, but it's as big a discount as you're going to get. We too like that it includes alcohol. We aren't big drinkers, but so few discounts include the drinks. I also think it's better than
DDP, at least for us. I'll take an AP room discount over DDP any day (unless of course they are offering
free dining or general public discounts comparable to AP rates.) We don't usually eat a lot of food in one sitting - so a full three course meal would be WAY to much for us. We greatly prefer to have our desserts, if at all, well after the meal is over while at WDW (say, a Dole Whip on the fly), and often only order one appetizer for the table, if at all.
It's quite simple to use DDE- you just give the CM your card.
We were able to use it just fine at the ALL-Star cafe, POR POFQ.
My one tip is that it never hurts to ask. DDE gives a discount at WOD store in DTD. You can also get discounts at some of the eateries there. It pays to consult the list you get with the card, since they make changes every so often. One restaurant we ate at, didn't normally accept DDE, but we also had REALLY bad service, so when we flashed the DDE card...they felt more inclined to give us a slight discount off of our meal. (We didn't whine or anything, we simply politely told the manager of the numerous mishaps that occurred during the meal: no bread, food allergy item (requested a food item without an allergic ingredient, but it was in the dish, had to send it back, and the meal was over before it arrived.), several of the dishes were very late arriving, wrong food item arrived....it was BAD service! Any decent restaurant would have taken somethig off the bill for mishaps as serious.
I know you're probably think you'd never forget but TWICE last year, TWICE (lol!) we ate our meal, even discussed the discount throughout the meal and how cool it is and left without using the card. 
BTW the second card has an additional charge now....$25 I think.
