We're a family of 5 and our kids are Disney adults also. We've been doing the dining plan for years, and it has been lots of fun to let the kids order anything they want, and get dessert (and in years past, apps too) with every meal.
We like doing a sit-down every day; it's a nice break and the atmosphere is always pleasant; so the dining plan did work for us from that standpoint (though we always had lots of snack credits left over).
But that said, the cost of the DDP has gotten so high that I wasn't able to justify it for this upcoming trip.
I booked our dining reservations first, then looked at the menus to see what we might eat, then figured out where we'd likely have counter service, and added it all up, and even if we all ordered the most expensive thing on every menu, I think we would have trouble breaking even with the dining plan this time. Add in the inevitable unused credits left over and it would come out as a loss. So we decided to go OOP this trip.
But everyone's advice to you so far, to look over the menus and use one of the calculators is smart, because everyone's situation is different. It just depends upon what your family wants out of this trip.