I've used it, too. It was very easy to use. The only problem I encountered was that the Honest Kids Punch was not listed as an option on the menu. I had to go through the regular line to buy it.
We used it for the cheese fondue also. I had a problem with the card reader on one, but moved over to the other one and it worked just fine. I did like how fast it was to use. The only possible issue I can see is that you cannot pay with cash.
I used one last year. It was fine, but I didn't find it to be any faster (or slower) than a human cashier at any of the booths. It's a non-issue for me. Given the choice, I would prefer the human interaction, if only so I don't need to be stuck behind people who don't know how to use the kiosk.