As everyone has said, it really depends on your family. We are a family of 5, 3 of us are considered adults, 1 a child and one is a toddler still. We have purchased the dining plan the last 3 visits and have it purchased for our upcomming trip in March. As a pp said, it does, to us, make it seem like a true vacation. Yes, you pay for it, but all at once and we no longer sit there and worry about what is cheapest to eat, telling the kids no to snacks, etc.
The only thing we didnt like at first was that you really need to plan your day around your TS meal. But, we have a system figured out and it works out great for us. We eat 1 ts a day, usually a late lunch or dinner. And 1 qs a day, sometimes we split these and have 2 ts a day by only using 2 credits each time. The snacks, we all try to get one per day, but somehow we always have plenty left over in the end and we just get some fun snacks for the way home.
So, it really depends on how you want to look at it. Yes, it is a pretty big chunk of money, and sure you can eat cheaper probably, but do you want to worry about it all trip or would you like to pay it all at once and not worry about it?