The dining plan doesn't save money or work for everyone. For us, it doesn't work out in our favour, we would be giving Disney MORE then we could be eating. We skip breakfast (coffee and some fruit that we already have with us), we usually share a counter meal for lunch as it's just too much food for us to eat on our own, and we usually do a sit down dinner. Sometimes we do sit down breakfast and then we usually do a late reservation for dinner. We have run the numbers and we are saving money by paying OOP.
Check out this link. It gives a fairly good breakdown of meal costs and if the dining plan saved/cost more
It depends. Basically, you need to do the math. Compare what you are spending per person per day on the DP compared to what you will spend at the places you plan on eating.
Every time we use it, we save money. Many people don't.