I don't think there is much, if any, savings. And then you are forced to only eat at specific places, etc.
I like to peruse the menus online before the trip, decide about how much each person will spend for lunches, dinners, snacks, etc., and then put the money for the day in an envelope. I always overbudget, too (usually I'll do $15 per person for lunch and I have three kids under six). I do $10 per day for snacks, and $20 per day for dinners (not including seperate "going out" money for when dh and I or my mom and I go out to dinners at DTD or whatever, which is a lot more than $20 pp).
It's my own
Disney dining plan, so I still get the feeling that everything is already "paid for", because I don't like to use the debit card at the park. It makes me feel out of control or something, like I'm not really keeping track of money spent, etc. I usually end up with a ton of "extra" money at the end of the day.
I also have a seperate envelope for souveniers, etc. I fully realize that this makes me look like an anal freak. And I embrace it.