In part it depends on how long you are staying. The mugs are $12. If you stay 6 days, that is basically one drink per day (yes, I realize that the mugs hold less than the 21 ounce cups, but you can refill them). For a shorter stay, it might not be a worthwhile.
Another factor is your room location in relation to the drink station. For our December stay, we walked right past the marketplace on the way to our rooms. We refilled frequently. Also, when we ate at Beaches & Cream our drinks were not charged as we had the mugs (weren't on DDP for that stay). Kids could get drinks at the pool also (yes, they swam in December....I didn't). At Saratoga, we will not go past the drink station between the bus stop and our unit, so we may not refill so often. However, given my fluid habits we will still more than get our use out of them. That way, snack credits can be used on something else!
For us, they are a great thing. I drink a lot of iced tea, and would get a couple mugs per day.