Swilphil, I was able to use the card in the World Showcase. I did not have too many problems in the restaurants but had most of my problems in the shops. Norway was especially bad. They were having a 50% off sale on some of their Helly Hensen clothing and the CMs did not know how to accept payment with the Rewards card. One young lady had to call her supervisor three times for help. Needless to say, if the sale prices hadn't been so good I probably would have given up and walked away.
The counter service restaurants were great. Absolutely no problems here. Just swipe the card, sign the receipt, and go. You get a printout with the balance remaining on the card after each use.
One problem we did have was at Mama Melrose's in MGM. The waitress knew we were using the card and that we wanted to use it to include the tip. When she returned she had a bill and told us that we could not add the tip to it. Like Earl of Sandwich, I just ended up paying cash.
I would definitely let the CM know you are using the card. That way if there is a problem, they can call a manager for help.