We like to eat the way the plans work (at least on vacation, lol.) So far, as long as we're eating at Disney restaurants,the dining plan is saving us money--when I price out what entrees we'd order, that alone comes close to covering the DDP cost. The snack & desserts at lunch & dinner tend to be "freebies" on the plan for us. We eat very few buffets, and virtually no burgers/hot dogs...and we tend to order from the high-end of the menus (LOVE steaks!) And yes, we like dessert on vacation. But even when I leave off desserts, we still save a little over paying cash.
I've kept all receipts on several trips and we've always saved 20-27% on food costs by using the dining plans.
If you don't like the high-end entrees, prefer buffets, or like to split a lot of meals then the dining plans are probably not the way to go.