My parents just got back from WDW a few weeks ago and raved about Randy's Mobility. They bring the scooter right to your room upon arrival and are great with customer service.
I would suggest going over to the disABILITIES Forum. One of the first threads is the disABILITIES FAQs. On Post #2 we have a lot of information for people with mobility and/or stamina problems, including lots of information about ECV rentals.. To get directly to the disABILITIES FAQs from here just click on the link in my signature.