Whether or not the DDP is a good value for you is really an individual thing. Sometimes it is based on each individual trip.
Our first family WDW trip was in summer 2008. This was before we became DVC members. It was very easy to see that the basic DDP made sense for our trip because we knew all the TS dinners we wanted, and all of them were price-fixed (character meals, Boma and Biergarten). We had a 3-year-old and a 1-year-old with us. The 3-year-old was on the DDP, the 1-year-old was not and he was free at the price-fixed meals. We ate CS lunches, and my little guy shared our food with us. At times I would get a yogurt for my dessert and give it to him. I knew up front what TS meals would cost OOP since they were all a set price, so I saw that the DDP wold save us a good amount of money. We loved it on that trip.
Our first trip "home" as DVC members was this past July. When I was making plans for our dining, I found that we had several more TS meals than CS meals this time. DH and I knew the basic DDP would not fit into our plans. We looked at the deluxe plan and decided that for 10 nights, it would be too much. We didn't have enough meals planned to cover all the credits, plus among the 4 of us, it would have given us 80 snacks!

I then looked into upgrading my ticket to an AP and then purchasing the TIW card. That is exactly what we did. After the trip, I added up the cost of all food, including any snacks we bought, plus the cost of upgrading my ticket to an AP, plus the cost of the TIW card and we came out close to $200 ahead of buying the deluxe dining plan for the 10 nights. So, as it turned out, this was the best option for our July 2010 trip.
Our next trip will be in July 2012. We will be going to our home resort (BWV) so we will be booking this coming August. With the new option of segmenting a reservation but not having to change rooms, DH and I are considering the basic DDP the first night, then 3 nights of the deluxe plan, then 1 night with no plan, then 4 nights of the deluxe plan. Right now it looks like that would fit our plans, but plans can and probably will change. I will just do the same thing I did last time we planned our trip. I will wait until I make our ADRs to see if we should add a dining plan, or multiple dining plans. I have also read several reports of problems during the trip when using a segmented reservation, so it may not be worth it to segment. The last thing I want to happen is going to the front desk one morning to "check-out/check-in" for the new dining plan, and having to wait a while because of a compter glitch or something. We like to be at the park before rope drop, and something like that could alter our park touring plans significantly. So it may be better to stick with the AP upgrade and TIW card just for the peace of mind.