OP, If you like to plan, then you really can figure out dollar for dollar if the plan would really be worth it.
You can find menus with prices on WDW Magic
http://www.wdwmagic.com/menus.htm
or On All Ears
http://www.allearsnet.com
All Ears also has a break down of what restaurants require two dining credits and what require one.
If you have an idea of where you would like to eat and what sort of dining preferences your family has (do they like to eat steak and expensive fish dishes while on vacation or are they happy with chicken, pasta and vegetarian dishes) you can figure out what is best.
It takes a few minutes, but you can make a good guess as to the best way to spend your money.
For us it came down to we like to try different restaurants at WDW, we like to have at least one sit down meal a day where we are served and we like the convience of not having to worry about the cost of the entree that we order.
So I sat down and looked at the menus, and knowing most of the places where we will eat and what my family likes to order (more steak and fish) and even with having to pay the tip outside of the plan and not having an appetiser included we will still save a couple 100 dollars. I also figured out that if we were to order the less expensive meals and eat at more buffets than actual full service sit downs we would save by not having the dining plan.
In the end, this is our only vacation this year, we want the conveince and we want to feel relaxed and free to order what we would like. So the dining plan works out best for us.
Have fun planning. I am sure the answer will become clear for your particular circumstance quickly.:wave: