For us they are well worth it. Both my mom and I drink a ton of coffee/soda/Powerade/hot chocolate, etc while at the resort. Usually two or three cups of coffee each morning, soda or Powerade (when available) when we return to the resort midday and then hot chocolate (me) and coffee (her) once we go back to the resort at night.
Plus we use the mugs everyday once we return home.
So for us personally it is money well spent. I guess you just have to figure out how many fountain drinks/cups of coffee, etc you think you might have during your stay and see if it makes financial sense for you and your family.