My personal experience:
I just got back from WDW 10 days ago and rented a
scooter from Walker Mobility. When I first saw them listed here on the DIS I called and spoke to a "real live" person right away, and she was able to answer all of my questions. I hung up, thought about it a little more and then called back and booked the scooter, and gave them my credit card number.
On the day we arrived, my scooter was already there, behind the Front Desk. It was labeled with my name and had the battery charger and info sheet with it. The fact that I did NOT have to be there for drop-off and pick-up was huge for me; I did not want to have to spend time waiting to sign for the scooter; I trusted both the company AND Disney, realizing that they've done doing this practice for quite some time now, and felt it would go smoothly. It did.

In fact, on our last day, the woman had told me that if I had the scooter back to Guest Services by 3pm that day, I'd only get charged for a half-day rental. Since we were being picked up to go back to the airport at 4pm anyway, it worked out very well for us.
We will definitely use them again on future trips.
EDITED TO FURTHER CLARIFY: The woman at Walker Mobility had told me in advance of my trip, that if I'd have the scooter back to Guest Services by 3pm, they'd only charge me a half day. I told her right then and there, before the trip, that I'd have the scooter back by 3pm. This wasn't something I decided at the last minute to do.
