I vote a resounding NO on the refillable mugs. Unless you drink soda 24-7, and needed it every hour on the hour, the mug really isn't worth the money.
Soda is served for free in Tritons/Lumiere, Parrot Cay, and Animator's as well as Palo. You get it at free at dinner, and I even got it at the sit down lunches in Tritons (I am not a buffet person, so I would go there for lunch).
Really the only place you actually pay for soda is at a bar or poolside, and the drink station was plenty for me. My advice if you like the occasional soda, get a 12 pack for 3 bucks before you board and put it in the cooler in your stateroom. You figure for 20 bucks for the mug, you have to drink approx. 8 sodas in a 4 day cruise just to PAY for the mug, let alone get your money's worth (that is figuring with the $2.50 price onboard). The drink station has tea, juice, lemonade, water... all better to drink when you are out in the sun, then soda.