The dining plan alows you the following per day:
One Snack (I usually use mine for a muffin, bagel, or fruit for b-fast, but chips, water popcorn, soda etc, are also acceptable)
One counter service meal (this includes the meal, drink and desert: this is usually your burger and fries places, but there are a few more inventive counter service meals if you look)
One table service meal (including a non alc/specialty drink, an appetizer, and entre and desert...yes that is per person. This is where the
DDP pays off because the resturants at Disney, while amazing, can be quite expensive)
If you are just a burger and fry guy and dont eat that much you may not want to DDP however if you want to eat at the sit down resturants every day, it is almost a must. Be aware though that you will need reservations and some (very few) are booked 5 or 6 months in advance.
We always do the DDP and as much as I eat, I never am wanting more.