I personally think the DDP was one of Disney's greatest ideas. IMO it gives guests an illusion of an "All-inclusive" vacation" so you never have to worry whether or not you'll run out of money for food. I can see why people love to go on cruises. Secondly, we tried restaurants we thought we would never tried in the past. Finally, it is great deal, like someone described it before, it is like getting a snack, a CS, and a TS for the price of a TS.
Last year we went during free dinning a were so impressed with the DDP we couldn't wait to book our next trip whether we had to pay for it or not (thank goodness for that April 1st announcement). I sit and laugh how some people are ripping the plan apart. (Too much food, too flexable or whether the servers like it or not)
Now for the Disney side of things. The Free Dinning promotion was a success be cause it filled rooms, empty rooms don't buy tickets and souveiners. The DDP is a much better plan (both economically and in practice) compared to past Dinning plans. It's the best word of mouth advertising Disney could have ever hoped for, the first thing I tell people at work now is about the DDP, Not the rides or the characters. I think it swings more people to the resorts vs. off-site.
Disney has set very high standards for itself, and the DDP has surpassed it.