My family just returned from 7 nights (buy four get three rocks!) with the dining plan and for us the DDP is more than just a cost saving tool. We view eating at WDW as an attraction just like any ride or show, and the DDP takes a lot of stress out of dining. With the plan we can order anything we want and not worry about the price. For us it makes the dining experience more enjoyable and even relaxing.
In planning for our trip I did an extensive cost comparison (I basically planned every meal we would eat with and without the plan). Long story short, we could save about $70 by not using the plan, but with the plan we would get a lot more food. We loved having the plan and we used every single credit.
My conclusion about the plan, you have to like eating at a TS every night, and you have to like planning out all of your ADR's in advance and sticking to that schedule. For us it is great, but it is definitely not for everyone.
One tip for extra value - if you have kids(3-9), the plan does not distinguish between adult and child counter service meals. For example, we had 2 adults and 1 child. We could get an adult breakfast CS meal and two adult CS lunch meals every day. This helped spread out the food some, and gave us a good meal for breakfast. The only restriction on CS is that you can't order more adult meals than adults in your group, i.e. we couldn't order 3 Adult CS meals at one time.