The mugs are approximately $12.00 each and you can refill them as many times as you want at your resort only. Whether or not they are a value to you depends on how many trips you are willing to make to the refill station.
In our case, it didn't make sense to purchase the mugs. There are four of us in our group so just purchasing the mugs would have been a pretty penny. Also, we stay in a moderate resort so a refrigerator is provided at no extra cost.
We drive to WDW so we are able to bring along Cokes, water, milk, etc. We simply put our drinks in the refrigerator and are good to go.
We spend the majority of our time at the parks so the only time we are at our resort to refill them is late in the evening.
Just doesn't pay for our family to purchase the mugs but I can see how they would be to others.