It's pretty much a guarantee if you use all your credits for the week, and order something average priced on table service menu (including dessert and non-alcholic beverage) and ordered an average quick service lunch, you'd spend less on the DDP then on OOP.
Example of a Day at Epcot:
Lunch:
Liberty Inn:
Chicken Sandwich combo meal (includes fries), apple cobbler and a soda = $12.07 X 6% - 7% (depending on county) = $12.79 - $12.91
Snack:
Soda/Juice = $2.09 X 6% - 7% = $2.22 - $2.24
Dinner:
Rose and Crown:
Fish and Chips, Vanilla Custard and a soda = $23.57 X 6% - 7% = $24.98 - $25.22
Total
$12.79+$2.22+$24.98 = $39.99
or
$12.91+$2.24+$25.22 = $40.37
So even going fairly low end (there are definately more expensive CS and TS restaurants), you're still paying less OOP. Going to places like Coral Reef or Le Cellier for dinner the night at Epcot would just make the cost event a larger difference.
In the end if you figure out how much you want to eat, you can back into how much you'd spend average per meal/day and then compare it to the costs of the plans. Some days you might break even, others you'll save a lot.
The amount of food is a whole other subject, if you're one not to waste anything, then you'll probably find yourself rather full.
There is also the part about planning, being on the DDP or not, doesn't change the fact you need to make reservations in advance, this is most true to popular venues. From the other planning perspective, some people like to know their food costs are already taken care of, not wondering how much to plan out etc.
Think of it sort of like a cruise. It's all inclusive, but the food suffers a bit (some lines are better then others), but you almost always have the option of a restaurant where you can pay a nominal amount the eat in a finer dining place on the ship. Some people don't see the point, but usually the food served in these nominal fee restaurants is far superior to the rest of the ship, the service is kicked up a notch and because of the cost (refer to people not seeing the point) it's not usually as busy.