When I went in Jan and again in May I didn't bother with the refillable mugs because I was doing split stays both times and didn't want to buy two mugs each trip. However in Sept we spent the entire week at POP and really got our money's worth out of the mugs. I am a very early riser so would get up and walk around the resort filling our mugs while my son and D-I-L got dressed. Then we would run back by and refill them before we got on the bus. The empty mugs went into my D-I-L's backpack and when we came back that evening we refilled them again before going to our room. Sometimes my son would run down a final time before we went to bed and get refills. We were able to get sodas, coffee, hot chocolate, apple juice, and slushes, by the way.
In Dec my mom and I are doing a split stay at POFQ and CSR. My mom has already said she wants the mugs as a souvineer, besides which she is a big coffee drinker. I am undecided at this point if I want to buy two mugs (which I'd have to do since we are doing a split stay). I think Disney should have a program in place where people doing split stays could trade their mug in for a new one when they check into their second resort. I don't mind buying one, but don't think it's fair that I can only use it a few days and then have to buy another one just because I switch resorts. It should be good for the length of your stay at WDW. The person who stays at one resort for ten days only has to buy one mug, but those of us who like split stays have to buy two or three to use for the same length of time.