You buy the refillable mugs at the Trail's End Buffet. They can be filled there with any hot or cold beverage. My favorite is the pink lemonade! :-D
You can also fill them at the Meadow's Trading Post - but only with hot beverages (coffee, tea & hot chocolate).
I think they're worth it! We bought ours and every year we bring them back with us and use them again and again! (The CM at Fort Wilderness told us that they were good "for a lifetime" when we bought them).