We've rented a few times with Discount Mobility\Florida Van Rentals (they seem to have multiple names). Overall, they were fine. They are the best price that I could find.
They usually drop off and pick up the van at the valet parking for the Hyatt Regency at MCO\OIA. This works ok. Sometimes, you can get a valet to help you with the bags to your airline.
The pick up hasn't really gone too smooth. The last time, they were over an hour late to drop off a van at the Boardwalk. I would say pack your patience with them.
The vans have been in pretty good shape. The last one we rented had a fold-up ramp and was VERY loud. We are used to a ramp that rolls up under the van (Honda).
We are going to try Rainbow Wheels (now called Wheelchair Vans of American, what is with the name changes?) for a trip in June. It is more expensive than Discount Mobility, but they seem to have nicer models and I want to try the curb drop off and pick up. I'll post back with our experience.
WLL