I find the mugs at BWV a bit prohibitive. You can purchase them at the Boardwalk Bakery and get them refilled at that location plus the pool bar and (I believe) the Belle View Lounge at the BWI. IMO they are more of a pain than a plus at the BWV because you usually have to wait in line to get them refilled. At other resorts where there is a food court you simply refill them yourself at the beverage island. Also, as the previous poster mentioned, you will have a refrigerator in your villa and it will be more cost effective to stock up on your beverage(s) of choice and keep them there.
Just my $.02.