I went with Deckert's when I rented one for my mother, thanks to recommendations on this board. They are awesome. It was so easy -- the scooter was waiting for us at the hotel when we arrived and we just dropped it at the bell desk on the way out of town. I was also impressed with the price, especially given the quality of the scooter.
I highly recommend taking some time to practice driving it around, especially backing up, before heading to the parks. The first day we were there I was convinced that we could not survive the whole trip without injuring a child, but by the end of the trip my mother was driving it like a pro. It just takes a little practice, and it helps if people are there to keep kids from darting behind it when the driver is trying to back up out of a parking space. Or better yet, put it in neutral and push it backwards.