You can't get milk with them....but you can get coffee, pop, and maybe juice (although it might just be some kind of punch rather than "real" juice).
It's basically what you can get at any fast food place (McDonald's, etc) that has the customer fill up their own drinks.
As for being worth it---I think some of it depends on what resort you stay at, and where your room is located. For instance, we stayed at POR our last trip, which is pretty big. We were in building 18, which is very close to the food court. We did get a re-fillable mug, and it was very convenient to walk to the food court to get it filled. However....for someone staying in one of the buildings a ways from the food court...it might not be worth it. I personally would not get one if I was in a building far from the food court.