Okay, here goes.....technically you are supposed to use your mugs at the resort you bought them at, for that particular stay. Say you are staying at POR, you buy mugs there and you use them for that stay only.
This being said, some CM's seem to be telling people that the mugs are good for any stay at that particular resort. I choose to buy a new one for each trip.
Now, here it gets really murky. There are a lot of people that use any type of mug at the resort soda bar. Mugs from previous stays at that resort, mugs from other resorts, and even 7-11 type mugs!! If you are using a non-Disney mug you are actually getting your soda for nothing! To me this is the same as stealing. And don't tell me that with the money you spend on the resort it is okay. Oops. I don't mean you, I mean that in the general sense of the word you!! This is a VERY heated topic on these boards. Do what you feel is best for you and your family. Most feel that you should get a new mug for each visit. We do this instead of another t-shirt as a souvenier!!!