For my family, the mugs are not worth it. We purchased a couple for our first stay at CSR several years ago, and I felt like I made myself run to the Pepper Market or to the pool side bar to get refills. I actually thought the mugs were rather small, and after filling them with ice, there wasn't alot of tea or soda. They did make a nice souvenir, though!
Our family rarely eats at the food courts at the resorts, and we use a grocery stop to stock up on drinks for the room. We did buy the refillable mug at Typhoon Lagoon, however, and I thought we probably broke even with it.
I can see how the mug would easily pay for itself if you plan to eat at the food court several times or if you know that you will make several trips to the food court for drinks or if you will spend lots of time at the pool with easy access to the pool side refill station. Our last few stays have been at CSR or AKL, and, for us, we just weren't willing to run back and forth to the food court for a refill everytime we got thirsty.
Just my two cents!
--dizneyfan