As others have said, it isn't a purely dollars and cents decision. It also has a lot to do with what sort of vacation experience you want to have.
We love the deluxe dining plan because it suits the way we eat on vacation. We just took a short non-Disney trip for a few days before Thanksgiving - in 3 days, even during the drive, we didn't eat one fast food/quick service type meal. We just don't like the hassles of that kind of dining with three kids, and that's even more true in places as crowded as Disney! I do cost comparisons for every trip, but don't even take the basic or quick service plans into account because they just don't suit us.
It does take more time out of the day to do multiple sit down meals, of course. If you (or your kids) are inclined to see dining as a necessary evil that interrupts park touring, the DxDDP isn't likely to work out. If, like us, you see dining as part of the whole vacation experience, it is a lovely addition that really gives the trip an all-inclusive feel. We have character breakfasts that we'd never pay for OOP because we're very light breakfast eaters ($30 for fruit & coffee just doesn't make sense), but on the plan we don't worry about that. And we have signature dinners most nights and very much enjoy ordering a variety of foods we can't readily experience at home in our little midwestern town.