I'm glad you brought up the 18% tip. In theory, TiW is really only a net 2% savings, and that's not taking into account the $75 membership fee.
I'm with you 100%. We've used the DDP on the last 6 trips. Each time we've kept the receipt from every TS, CS and Snack Vendor and tallied them up at the end. Savings varied, but there was always a significant savings. And it didn't make life any more inconvenient for us - we didn't adjust our plans at all to accommodate the DDP, because we would have been eating at most of those places regardless. I also don't adjust what I order based on the DDP - except that I WILL get something more expensive on the plan than I would if I were faced with that choice outside of the plan...but it's still something I like.
People will argue that while you do indeed spend less money on the plan than you would purchasing those same meals with cash, it can be a false savings if you wouldn't normally order that much food if you were spending cash.
For example, we always drink water with dinner at home (when we go out) and never order desserts. No desserts because we're usually full. No water because I'm a tight wad. So, if I were to dine in the same fashion at WDW, I'd definitely spend less in cash than the DDP would cost me. However, I'm not interested in pinching pennies when I go to WDW - because I've spent all year pinching pennies just to get there.
So, if that means my wife and I can actually have tea or soda with dinner, then order a couple of desserts, try a few bites of each, then toss them - so be it. I'll "save" money by getting the DDP, because if I were faced with those options on an out-of-pocket basis, I'd never allow drinks or dessert because my thriftiness would kick in. With it being pre-paid, I can have whatever I want and eat as much or as little of it as I want, and pay a flat rate.
It's a gimmick, and I've fallen for it. But at least I know what I'm getting and what I'm paying for - I'm not naive about it like many people who get the plan and then eat like birds. They're losing big time.
Edward pretty much nails why the DDP works for some of us, but not everyone. I think one thing some of you are overlooking (and it's understandable, given the venue) is that not all of us can swing multiple visits to a Disney park in a year, and may go many years without a visit. To wit, our last trip was in 2002. Our next trip will be this January.
So that makes it a very special occasion when we get to go. As would be the case with most special vacations, we're not going to be eating as conservatively as we normally would, regardless of how we're paying. We're both big on seafood and more exotic fare and we love fine dining. As such, most of the counter service options leave us cold. So using the DDP and booking most of our dining at table service restos (and, under the deluxe plan, even a couple signatures --

shocking, I know) is one of the attractions of the trip, not simply a distraction from rides, as I imagine a lot of folks see eating in the World.
My wife is an engineer and there are few things she loves more than generating spreadsheets.

We actually sat down and put together a spreadsheet listing every single item we would order at each of our intended ADRs under the DDP, including the cost of tips and drinks. Also included any snacks we knew were must-haves. In another sheet we calculated what we would order if we were paying OOP. Once the 2011 DDP prices and details were announced, we compared the sheets and came out significantly ahead under the DDP, even though we'd made more conservative decisions OOP. At that point the DDP was a no-brainer.
Nevertheless, I can definitely see how this would not be the case for everyone, especially if you're not a foodie and simply want to cram something in and get on the next ride ASAP. I get that.
But it's a lot like the people who don't understand why anyone would want to "waste money" on a deluxe or even a moderate resort when Pop Century or an offsite hotel will do the job. And they have a point because they can probably afford to go more frequently. We did that when I was a kid, but when I was in college, I had my first experience staying on property in a moderate and later a deluxe resort, and I knew I could never go back. Does that mean less frequent trips? Yup. But does it also mean I enjoy those trips more? Hell yeah. Different priorities.
Last, If they failed to plan , being a member of WDWMAGIC......shame on them. There is way to much info here to make a mistake like that.
Yeah. I genuinely feel bad for the folks who barely even understand what an ADR is, much less bother to make one, and who walk into just about any TS restaurant and get a hard dose of reality. But if you're hanging around here, there really is no excuse for not knowing the score. Make your ADRs...six months out, if you have that luxury.