Our last two trips we've gotten one as a family. We usually fill it with coffee twice in the morning (if someone is up early enough to make two runs, with our kids that's usually true), and then someone in our group will have a cold drink or two in the afternoon when we're at the pool. So we get 2-4 drinks per day in it, which means it pays for itself pretty fast (although if we didn't have it we probably wouldn't have the afternoon soda--none of us drink much soda at home).
I wouldn't buy them for everyone in our group, I don't think we would use them enough to make that worthwhile. And we've never done the dining plan.
ETA: I think it also depends on what resort you're at. We were at POFQ this trip and so every room is pretty close to the food court. If you're at one of the larger resorts and might have a longer walk, that's a factor too.