Don't know about you but when we travel with five our TS meals are in the $200 range. That's approx $40 off with DDE per meal, which makes the card a definite savings for us.
Looking at just the cost of the DDE, I figure anything after 2-3 meals is a savings. It doesn't work for counter service inside the parks and I don't think it's good for pool bars either but you can use it for beaches and cream at the BC/YC in addition to standard TS meals and resort food courts.
I get TA discounts, which are slightly better than AP rates but the cost of
DDP, added to a TA or AP rate, is much more than using the code for room only and our DDE discounts on dining.
Oddly enough, I am having just the opposite ticket quandry this year, trying to decide whether to buy DH a ten day MYW by upgrading a 7 MYW to non-expiring or whether to buy two 7 day MYW because he has a work break of 10 days in between our first week of vacation and the third week in wdw but probably won't go back until Fall 2009, making an AP more expensive I suspect.
We could use 7 days, then upgrade another 3 days and no expire (because it'll be after the first 14 days before he can use them all) but it still isn't enough park days that way, unless he buys two non expiring MYW 10 day and saves 3 extra days on each, but that's $191 extra just for non expiring, making an AP still a consideration.
Ever feel like a dog chasing his tail?