This was a puzzling question we had when planning our next trip, because it would be the first time we would be going for more than 4 days, and the first time we even considered doing it. What I did was make a spreadsheet with each day planned out, where I would eat for TS and QS, the items I would most likely choose to eat, and dig up a menu with prices from here or allears and go through the math. I think I used an average price as a snack, as that will greatly vary and I don't know what I will want at any given time. So after I did that for the both of us, accounted for gratuity and tax, I could figure up how much we would spend without it, and how much it would cost to add it to our reservation. Lots of planning but that is part of the fun, right?
So for our weeks meals it added up to about 580 dollars if we were paying out of pocket, and about 530 dollars to add the dining plan to our reservation. So we saved about 50 dollars.
Now, one needs to question if you can actually eat that much food, how much you actually want, the time cost of waiting and sitting at a table as opposed to attractions, etc, before one can really say if it is better or not. In this instance, it was our first long trip there, and we wanted to really do the whole 9 yards, and my wife really likes the restaurants theming and atmosphere, and it is a good way to relax.
If it were me, I would go quick service only because I really don't get that much additional value out of a TS restaurant, and would really only need to visit a couple over the coarse of a trip to be happy.
So, I think if you are looking at this objectively, looking at just the numbers, I would say that yes, it is worth it. Subjectively, however, it is a whole different beast and dependent entirely on you and your families eating habits, preferences, and what you really want out of the trip.