My family unfortunately has the bad habit of drinking more soda that we should and there is nothing that compares to fountain soda. If you are staying at a Disney Resort you should consider purchasing a Disney Souvenir mug to help cut costs on soda. The cost of the Disney Souvenir mug was I believe $14.99 when I was there at the end of February. The sad part was I remember in 2008 when it was only $11.99. The cups are intended to be used only during the stay when you purchased them, but I cannot tell you how many people I saw using mugs from years past. Use your own moral compass if you choose to do that.
The mugs can be purchased at the food court of the Disney Resort you are staying at. Many of the resorts have the same mug. The mugs are currently available with blue, yellow or pink lids.
Disney carries Coca-Cola products which can used in the mug along with coffee, tea., and hot chocolate. Sorry Pepsi fans there are no Pepsi products at WDW to be found that can be used in the mug. Unfortunately, the mugs cannot be used for milk or juice.
You may ask if it is worth it? To me it was. I spent quite a bit of time at the resort and soda machines near my room were selling bottles of soda for $2.50 apiece.