For us as two adults it certainly was worth the $50 over a nine day trip. WE had a table servie breakfast almost every morning at Olivia's and one other table service meal during the day. If you figure the 20% discount pays the tip, we saved about about $125, or $75 over the cost of the card, with another trip planned in December. The AP dining discount are limited to mostly weekday lunches @ 10%, the DVC discounts are a little better, but they are mostly 10% or 20% for lunch at Marrakesh, Chef's and Nine Dragons.
I guess it really will depend on your personal dining habits and your length of your vacations over the course of a year.