My upcoming trip may be the one and only time we are even entertaining the dining plan. We will be a party of 4 adults, 1 child, and 2 babies, split up over two studios. We are contemplating buying the deluxe plan for the members in the room with 2 adults and the child. We would then split the adult credits over the 4 adults, and use the extra "child" credits to split some quick service meals.
Outside of some specific creative "loopholes", I don't think the DDP is worth it. I basically look at it the same way I look at an extended warranty (except the reward is actually way worst). It essentially puts a fixed cost on your dining budget, but your paying a premium for that insurance.
At an all inclusive resort/cruise, they've run the numbers and charge more for the all inclusive package than the average person will eat. The house wins on average, but an individual can still personally "beat" the house if they are large eaters. With the dining plan, this is not possible. They cap out how much value you can extract. So while it is still possible to "save" money using the dining plan, your not going to beat it by a large dollar figure even in the best case scenarios. However, the possibility of losing to the house is much more likely (see all of the blog posts about having left over quick service and snack credits at the end of the trip). Given that, paying a premium to eliminate the minor risk that the dining plan would have been cheaper than paying out of pocket seems silly to me. If I'm spending all of this money on a Disney vacation, I have the extra funds to cover the slight possibility of spending an extra $50-100 per person on dining.