The Dining Plan is a deal if you take full advantage of it. For example, at sit-down restaurants, you are able to get an appetizer, entree, dessert, and non-specialty drink and it is all covered by the plan. With counter service, you get a meal, dessert, and soft drink. Also, character meals and the Fantasmic Dinner Package are one set price, regardless of the food you eat. The Dining Plan can be used for both of these, so even just for that, it's pretty helpful.
If you aren't planning on eating that much (or you are not going to character meals), it may not be worth it. But, if you will be or just like know you can eat that much, it's a great deal. We just got back from one week at WDW with two adults and two children. We had 6 days on the @ $38 per adult and $10 per child (under 10), for a total of around $600. We haven't figured out a full total for the trip, but by the middle of day 3, we had eaten $572 worth of food (that included two character meals, four counter service lunches, and dinner at Le Cellier, the steakhouse in Canada). And after that, we still had two more character meals, a dinner at Mama Melrose, and the remainder of our counter service and snack credits left.
The other way to look at it is this: Do you think you would spend more than $38 in food per day for an adult or child over 10? Do you think you would spend more than $10 in food per day for a child?
If you answer yes to either or both of these questions, get the Dining Plan.
When we went in 2002, we got a cabin at Fort Wilderness and cooked all our food. It was OK, but kind of a chore. This time, we got the dining plan, and didn't have to worry about having enough money for food. I really liked knowing that no matter what else we did with our money, we knew we were going to be eating well.