We got the DDP for the first time on our most recent trip (last May). It was three adults and one child (plus another child under age 3).
We got the dining plan for a couple different reasons: I was approximately 27 weeks pregnant and knew I'd want more time than usual to sit and relax in the air conditioned restaurants (plus eating was one of the things I could do since most rides were out). Normally we're very commando Disney travelers - parks from opening until close every day, no naps, no pool breaks, seldom take time for sit down meals. As a result, we hadn't gotten to experience a lot of the sit down restaurants we'd been wanting to try. So between my pregnancy and having two kids (one quite small) with us, it was a good opportunity to slow down a bit and take more time to dine.
I made a list of the restaurants we wanted to try and looked at the menus to get an idea of what we'd probably each want to eat. Because of the restaurants we wanted (several character meals, mostly dinners), and even totally disregarding desserts, I knew we'd at least break even by getting the DDP. Additionally, there were instances when I knew DH or I would probably like entree X but would settle for cheaper entree Y if we paid OOP.
When we got home I tallied up the receipts we had (several were missing). After totally probably 80% of our receipts, we'd already saved over $300 by using the DDP.
Yes, it was a lot of food. And we didn't eat, or only took a couple bites of, a lot of the desserts (although we were able to replace several CS desserts with bottled water or with fruit at WPE). Even discounting the dessert issue, we saved a lot.
We'll get the DDP again in the future, but will weigh whether we want a more fast paced trip with few sit down meals or a more leisurely trip first.