We did it for the first time in 2012 and I think we came out ahead. ONLY because we ate at places that made that possible. Boma dinner, Ohana dinner, Akershus dinner, etc. When ordering off a menu, we felt compelled to order the most expensive entree (and to encourage our oldest DD to do so) even though it might have not been what we wanted most. Many times desserts at TS and CS went uneaten and we had a fridge full of CS desserts by the end of the trip.
For our 2013 trip, we want to eat as we please and try some "lower priced" places too, so we are skipping the
DDP. We planned to get Tables in Wonderland until the price went up last week. Now I need to crunch the numbers on that some more. It might make sense for our nine night trip.
But I feel like the dining plan tends to make you eat in a way you otherwise wouldn't. It's great for prepayment, but that could come at a premium depending on how your family eats.