mysticcruiser,
It depends on what resort you're staying at. We stayed at POFQ, bought the mugs, and they were great value and the boys had great fun refilling them, which we did often (we were so near the food court). When we moved to WL we bought them one each there too but rarely used them because Roaring Forks was quite a walk from our room and it wasn't worth the walk just to refill the drinks. This year we'll be at BWV and I hear you need to ask a CM to fill it - with BW being big and a possibility that we'll be very far from the refill location I don't think we'll buy any this time. Also I'd feel awkward asking a CM to fill 5 mugs each time, just me, but it doesn't sound as easy as it was in POFQ, I'm guessing.
They make great souvenirs though and I'd recommend buying at least one just as a souvenir if nothing else.