I like to think of the dining plan like this:
Your average full sit-down meal (drink and entree) will cost you $40 (give or take, depending on where you go)... which is the price of the dining plan for each night.
Your average counter-service meal (drink and entree) will cost you $15 (give or take).
Your average snack will cost you $1.00-$5.00.
Based on this I draw the following generalization:
For the regular dining plan: If you commit to Disney to pay for a sit down meal each night of your stay, and pre-pay for this, then Disney will give you: One free counter-service meal per night of your trip, one free snack per night of your trip, and desserts for each of your table and counter service meals. This gives you a value of $15-$20 for each night.
For the deluxe dining plan: If you commit to Disney to pay for two sit down meals each night of your stay, and pre-pay for this, then Disney will give you: One free sit-down meal, two snacks, a resort refillable mug, and desserts and appetizers for each of your meals. This gives you a value of $50-$75 for each night, plus the resort refillable mug.
For the quick-service dining plan: If you commit to Disney to pay for two counter-service meals each night of your stay, and pre-pay for this, then Disney will give you: Desserts for each of your meals, two snacks for each night of your stay, and a resort-refillable mug. This gives you a value of about $5-$10 for each night and the resort refillable mug.
Now, obviously, this only works if your eating habits match that of the dining plan. For a lot of people these plans are too much food and would rather split an entree and dessert, they'd rather have appetizers instead of desserts, or they'd rather have an adult drink instead. For them, the extras provided don't have any value to them which makes it hard to justify the plan.
For some people (who eat very much like the dining plan prescribes), the plan provides a great value. I also like the fact that before I go, it's VERY easy to budget. The cost is fixed, it's easy to budget for, and once it's paid for I don't have to worry about anything else.
I hope this helps