We love TIW. We don't need a dessert at every meal (which we feel obligated to eat on the
DDP because it's "included"). Half the time, we split our meals. We are free to order whatever we want off the menu. On the DDP, we had to order the boys stuff off the kids' menus. Well, there's only so many chicken nuggets you can choke down; this way, we can order the kids whatever they want and still get 20% off.
As far as planning the cost of food (I have heard this argument FOR the DDP), I just figure the actual cost of the meal. For example: The Cape May breakfast is priced at $26.99 for adults and $13.99 for kids. I add up 2 adult meals and 2 kids' meals. I don't bother to add the tip because I know that if I plan for the full price of breakfast, I'll have plenty once the 18% tip is added.
I haven't read to see if this is still a perk or not, but the last time we had TIW, it covered valet parking if you were eating at a resort. At that point, TIW paid for itself in the first couple of days (I think at the time valet parking was $12).