We tend to have a TS dinner every night when we are on vacation, whether at Disney World or anywhere else. We also tend to indulge in dessert on vacation, so the
DDP works for us. It saved us money since we normally would have a TS dinner anyway. We had all-you-can-eat TS meals on our trip last July, making it easy to figure out which ones to pay OOP and which ones to use TS credits for. We used the TS credits on the dinners (all cost between $25 and $30 per adult at the time, without tax which is included in the price of the DDP) and paid OOP for the 2 breakfasts we had (roughly $20 per adult before tax).
If you look at the WDW menus, you will see that the most expensive entree at a 1-credit TS restaurant will run between $25 and $29. Usually it is steak. I love steak but would I order it EVERY night of my trip? No, I would want to try other things too! So from a realistic point of view, it is better to see what you would pay for a full day of meals if you were paying OOP, with the cheapest items on the menu, vs. what you would pay for the most expensive. This is easier to do with all-you-can-eat places since it is one price.
Personally I am obsessed with Disney planning, so I looked at the menus for the places we will most likely try on our 2010 trip (yes, next year and I am already planning the meals...like I said I am obsessed). I did the math with the least expensive things for each of us to possibly order (even though I know DH will not be ordering a vegetarian entree and these tend to be the least expensive, but hey you never know if he will change his mind, lol!) and the absolute most expensive (that NY strip steak and the most expensive dessert). Both ways, on a 10-night trip, the DDP will be saving us lots of money. Now, as I said, we are also people who enjoy our desserts on vacation so that is good to have it included, we love the character meals with our kids, and we always have TS dinners on vacation no matter where we are. We also tend to snack a lot while at a theme park so the snack credits come in handy since we like to enjoy different things. I estimated that the DDP will save us anywhere between $265 and $566 on a 10-night trip next July with the boys. The lower figure was with the cheapest snacks as well, and the higher one with the most expensive options of everything in every place we are looking to go. Realistically the savings will most likely be something between these 2 figures.
I hope this helps!