There are some places in WDW where you can't use your room key to charge, but to the best of my knowledge, Disney Dollars are universally accepted on property. You can even tip with them -- cast members can "cash out" Disney dollars at the end of their shift.
They work great if you have a Disney Visa and want to get points for everything you spend, and don't want to turn on charging on your kids' room keys. (Buying them with a credit card is treated as a purchase, and not a cash advance.)