To clarify, the refillable mugs are available at a WDW resort. They are about $10 now.
Each resort has its own mug. You can get unlimited refills on soda, coffee, tea, hot chocolate at the snack bars/food courts of your resort.
You cannot use the mugs at the parks, or at another resort. (I know some people do, and the CMs ususally don't challenge you, butm hey, people--let's follow the rules) For example, if you have a Caribbean Beach mug, you cannot use it at the Contemporary.
The exception to this is the All Stars. They are considered "sister resorts", so you can use your Movies mug at the Sports or Music food courts.
The mugs are good forever--you can use the All Stars mug you buy in 2002 for your next trip in 2005. You can also use the Caribbean Beach mug you bought in 1993 (if you remember to bring it)
A few words of warning.
Make sure you know where your room is in relation to the food court. At some resorts, it can be a long walk that you may not want to make.
Also make sure you understand which beverages are and are not included. My understanding is that juice and milk are not included. So, if your kids prefer juice to soda, they may not be a good deal for your family. If, however, your family loves Coca-Cola products or can't stand the tap water, they are probably a bargain.
Plus, they make a good souvenir.
:slurp: