We did the dining plan (1TS, 1QS plan) on our last trip and it was nice not to have to worry about what we were spending on our meals and we could just order what we wanted. I think we might have ended up with a credit or two of snack left so we just grabbed a few things before we left. I didn't like having to keep track of credits -- it was actually confusing when we were in France at the pastry shop and all we wanted to get was a few snacks but i think we ended up using a QS credit and some snack credits - throwing us all off.
The down side is that it is much more food then we typically would eat - a fairly big lunch (QS) a large meal and dessert (TS), snacks every day. This is not how we typically eat, so our next trip we will likely still do just as many TS & QS meals, but we will probably order a couple appetizers and split a meal and not do dessert, so with this situation I feel we will have more flexibility and won't spend as much and with the AP or
DVC discount we will probably make out better. As a comparison I am going to load the same amount of money onto a Disney gift card and use that for our "dining" plan.
I should add that it seems like the dining plan can be a benefit if you have Disney kids (under 9yo) and you are planning many character meals. From what others have noted - in this scenario the dining plan can be a little bit of a savings. I did a rough pricing of my family for our next trip 4 Disney adults and 1 kid. For the character meals we wanted and a selection of other TS and QS meals - there was maybe a $100-200 savings if we went with the dining plan. But if we go the root as noted above (more apps and splitting meals and DVC/AP discounts) we will likely ended up a little better.
The dining plan i don't think is really set up to save money but to give you the all inclusive feel. Even the mugs for us were probably not much of a savings due to the fact that we were in the parks more than at the resort and only got a few sodas and coffee. Despite that we love our mugs, use them almost every day so we would probably still get them for our next trip. It is one of those very useful souvenirs.