Agree with the above! Grab a few of the menus (posted on this site in the dining section), take a look at the pricing, and figure out what it would cost your family for a couple of days of Disney dining out of pocket. It's actually pretty quick and easy to do. Then compare that amount with what you'd be spending, per day, on the dining plan. (*Keep in mind that if you want any of the 2-credit table service restaurants, they are almost always a better deal out-of-pocket. Don't waste your dining credits on them.)
We got the dining plan last time (2 adults, 2 children), and even ordering the most expensive things on every menu and eating at the more expensive 1-credit table service locations, we saved exactly nothing. We broke even, paying the same for the dining plan as we would have if we'd ordered the same items and paid out of pocket.
In fact, the dining plan cost us more money than we'd have otherwise paid, because we ended up with more food than we'd otherwise have wanted to order or eat. (We tend to drink water instead of fountain beverages, seldom order dessert, and don't necessarily always WANT the most expensive entrees on the menu.) Had we gone without the dining plan AND ordered only the things we'd normally order, we'd have saved about 20% by paying out of pocket rather than using the dining plan.