I would never do the dining plan. Why?
1. Because restaurants are a hassle with our young children. Fun once or twice on the trip, but that's about it.
2. Because I would rather be on Space Mountain than waiting on a check/figuring out credits/reading a menu/waiting for my party to finish.
3. Because my first three days of a trip I am still so excited that the only thing I can manage to eat are half a turkey leg and a Mickey Bar.
4. Because for breakfast we usually only eat oatmeal, toast, coffee, etc. Can have those in the villa. Once or twice a trip we'll scramble eggs and have bacon too. But we're still fed and out the door well before park opening time if need be.
5. This goes with #2, but one thing I figured into the equation is the amount of time a full-service restaurant takes. With my kids, that's actually probably longer than the average family (and no offense to my mom and sister, they add a good deal of time on there too as they are pokey.) We'd literally lose hours of park time. And I love the parks. I love food too, but I just can't bring myself to eat that much.
6. I'm an epicurean at home. I cook absolutely wonderful foods, and we eat NO junk food at all. When I'm at WDW, I want my Mickey bars, coconut patties, turkey legs, and pizza on the Boardwalk. I never get those things at home.
7. I'd rather spend $38 a day on walk-up margaritas in Mexico and an eggroll in China. Down that margarita, head to Soarin again, and I'm good to go.
To each his own. For us, dining plan=too much time, too much food for our appetites.