There are free-refill mugs that are sold in the resorts. These run about $12 each and you can get free refills on coffee, soda, tea, hot chocolate & lemonade. You can only use the mug at the resort that you purchased it at (say a Wilderness Lodge mug you can only use at the Wilderness Lodge and not at any other Disney resort. Also, resort mugs are only good at resorts and not at the parks. You can purchase souvie mugs at the parks for about $5 (I could be off here), but these do not include refills at the theme parks (not sure about the one's for the water parks). Now the touchy subject is the reuse of mugs if you are returning to the resort you purchased them at. For example, if you stayed at the Lodge last year and are heading back there again this year. Some people feel that the mugs are good for one trip only, while others reuse their mugs. I personally reuse my mugs. I purchased a WL mugs when I stayed there in 2001 and brought it back with me for my stay there in 2003. After asking many cast members, I was told that I could use the mug there, since it was purchased there. These also make nice souvies. I use my Lodge one at work, another one in the car and the others are on display with some other Disney stuff. The resort mugs are TOTALLY worth buying, even if you are not at your resort that much. Think of it this way. Say you use it in the morning for coffee, then fill it with soda on your way back to your room for a break, then fill it again before heading to bed. That's like (again ballparking it here) $8 per day times how ever many days you're there and you can see you're saving some $$$ here.
Hope this helps!!!