When evaluating the ROI of the DDE card, one has to take into account the ticket element (ergo, do you really need an annual pass?) as well as your dining preferences.
As example, I'm a DVC member who is not a Florida resident. I'm taking an extended family party of five (three adults, two kids) down to WDW next February for six days, and that is the only trip we will be making there that year.
The cost of a 6 day park hopper for me will run about $246. A basic annual pass (with my DVC discount) is $334, and to get a DDE card, I need to buy it. The DDE card is $65.
So, my total "additional" investment for my trip to get access to the DDE discount would be $153 (the $88 difference between the 6 day park hopper and an annual pass, plus the $65 cost of the DDE card). To get a positive return on that $153 investment, I'd need to get at least that much (preferably more) in dining savings over our six day visit.
Given the 20% DDE discount, in order to make my investment back my party would need to spend at least $756 on seated meals over our six day visit.
Now many here might say "with five people, that's easy." But not so fast: I know from our prior trips that because we are staying in a DVC villa with a full kitchen, we will likely eat almost all every breakfast, several lunches and at least one dinner in the villa (using groceries we willl have delivered by an outside party upon our arrival). As a consequence, getting to over $750 in seated dining expenses may be a stretch, and even if we go beyond that, the DDE savings would likely be minimal (e.g., is the extra effort involved in buying an annual pass - which I would have to go pick up at WDW on arrival -- worth saving $50-$60?)
Bottom line - DDE works well for many....
...but not for everyone.