My family and I have stayed at the Caribbean Beach Resort almost every year
since it opened (1988-2004) and have re-used the original mugs since the first year. The staff members there have even made comments that it's great to see people come back {they can tell because the mug design changed about eight years ago or so}. We were told long ago that they were
good for life and were never told otherwise. We also have mugs from Animal Kingdom Lodge, All Star Resorts, the Wilderness Lodge, and Pop Century.
We have re-used at the Animal Kingdom Lodge and All Star with no problem.
You're not supposed to use other resort mugs though. Usually you can get
soda, iced tea, water. Nearby is usually cofee, tea and hot chocolate.
For the price of the mug, it's really a bargain if you add up all the drinks
you ususally consume on vacation. I imagine that DVC resorts follow the same policy.