We have done free dining three times; the rest of our trips have been paying OOP for our meals. Travelers are me and DD, who is an adult, and we stay at value resorts so when we have free dining we upgrade from QSDP to DDP. We don't do character meals, but do some buffets (Biergarten and Boma) every trip. Here's what I've learned about the DDP.
1) Until recently I was a Pooh-sized lady who could put away a considerable amount of food in one sitting (20-24 chicken wings on wing night, for example)- but I always find the portion size at Disney to be MORE than generous. The DDP is a LOT of food for us. When we have the DDP, DD and I will share a bounty platter for breakfast, a larger "meal" for lunch, and then each have our own TS for dinner, as well as our own snack. This way with free dining we are not paying for any additional meals. This is ALWAYS enough food for us. In fact, we tend to eat the same way when paying for food OOP.
2) Keeping track of what we ate, each time we've done "free dining," it's cost us more than if we'd made the same food choices and paid OOP. We don't try to maximize the dining credits by eating the most expensive choices, but we are both happy to order entrees such as scallops, lamb, duck, etc., and these are usually some of the more expensive entrees. The three times we've done "free" dining, it's cost us about $250-$300 more than if I'd booked a Disney value hotel at a discount, purchased discounted park tickets, and paid for the food.
3) I had free dining for our upcoming trip (Aug 2018). It was going to be about a break-even proposition for us, so the fun and convenience won out. WHY did I think it'd be break-even? Because the DDP now includes adult beverages and two snacks a day. We enjoy a drink or two so being able to get one beverage at lunch and one each at dinner adds up fast, and that's what really makes "free" dining a break-even proposition... that and the second snack credit.
HOWEVER.... notice I said "had" free dining. We decided to give it up, for a variety of reasons. I was very conflicted about having free dining, as I had bariatric surgery 18 months ago. I just don't eat the way I used to, which is a good thing! However, it means I eat very, very little. Any entree Order is going to go to waste; I can eat about 3/4 of a cup of food at a sitting, not too many times a day. Eating 3 times a day keeps me very full; there's no way I can include one snack, much less 2 a day, and DD doesn't eat that way, either. I realized that, while FD would allow me to order whatever without worrying about wasted food vs. cost, there was no way we'd get the full value of the DP, and we have no interest in bringing home a ton of Goofy's candy to use up the credits. When I found a TA offer for 6 nights at AKL with park hopper for the same price as the FD offer, we decided that would work better for us. I have some gift cards and lots of rewards money from my Disney VISA, so we'll use that to pay for our food and drink. That's going to work best for us this trip.
SO... Free dining, and/or the DDP, is a good deal only if it works for the way your family eats. Even then, like us you might find that you would spend less money paying out of pocket. There is no hard and fast rule about whether or not the dining plans are a good value... you have to consider how your family eats, look at menus online, run the numbers, and THEN decide.