It depends on a lot of things. If you aren't big eaters and like to occasionally share a meal, than you can save by not doing the Plan. Also, if you would never order dessert than that also figures into it. If you are like me and two of your adults are 11 and 13, thats a factor. If you eat TS more than half your trip, your kids are really kids and you would normally eat the way the DDP works (cs meal, ts meal, snack, desserts) by all means I think the DDP is great. For the convenience alone.
I go both ways, sometimes I just get irritated when my 11 year old pays the same as I do and ends up not ordering half of what we pay for. And also now that the gratuity is not included, its definitely not as good of a deal. Yet, for our next trip we are doing the DDP, last trip we didn't and my 13 year old eats so much, I couldn't keep him full! His appetite tanked my best laid plans, had we been on the DDP it would have been great, he could have eaten what his 11 year old sister didn't! So next trip, its 15 nights on the DDP.
I think if you spend the time to figure it out based on your families needs, wants and habits, you'll be able to come up with what is right.