Refillable only during your current stay, at the resort at which you bought it. But, I have about 17 of them. I buy them at every resort, each time we go to WDW. They are wonderful souveniers. I have my morning coffee everyday in one of them. I keep one at church for my coffee there. I can't see why people get so upset about paying $12 for a mug and 'only' being able to use it at that particular resort, for that trip only. When I think of the money we spend at WDW, it seems to be a drop in the bucket. And yes, I do realize how much it costs if you have a family of 6...$72!!! But why do people balk at that, yet spend that much buying teeshirts for the family?!! But that's just my opinion.
At $2 a pop, at least, for drinks in the food courts, all you have to do is have 6 of them and the thing has paid for itself. And we do that in about 3 days.