I think the current pricing for the refillable mugs at the WDW resorts is borderline and if it goes up much more, there will be a big drop in people purchasing these mugs. At my last visit in June, a large drink was around $2.50 (I think - I remember using that amount to decide on the mug or not). If you are there for a 7 day vacation, you have to refill it 6 times to break even. If you do not eat at the food court just about every day, this isn't too likely. If you are there for a shorter trip it is REALLY unlikely.
If you are a soda addict and are willing to head over to the food court over and over to fill your mug - then its a better deal. We tend to stop and pick up a case of diet coke on our way to our resort, so we are only using the mug when we have a meal at the food court. About the last thing I want to do after walking all over the parks is hike to the food court just to fill my mug! My last trip was for 10 days, so I figured we had a decent chance of going to the food court 6 times. I tried to keep track, but I think we did - it might have been only 5 times.
If the mug cost $25, you would need to use it at least 10 times and $33 would be 14 times. At a certain price point, it starts to make more sense to just buy individual drinks when you go to the food court.