I have rented from them twice - fall 2014 and again december 2015. Both times I have been pleased.
Fall 2014 was the first time I rented an ECV and our trip was a split trip with a cruise in the middle. I needed the ECV for the bookend WDW stays.
The first I rented from Walker as I wanted them to show me how to use it (having never used an ECV) -- so renting form a vendor that has to pickup and dropoff with the customer seemed a good idea, and based on reviews Walker seemed to be one of the best of that group. Their service was great. I did find it a bit challenging with respect to having to meet them for drop-off and pick-up -- having to schedule that into the day and find a time they and we were available created some challenges (for both receiving which was on our arrival day after flying from canada, and for giving back because they only had one "window" available and it conflicted with our plans -- we changed the plans so we could meet Walker). Not insurmountable, and not a reason to not rent with them again. Just something to be factored in when planning. For a first time
ECV rental I think it was a good idea and the benefits outweighed the downside.
However, I did find the
scooter a bit of a challenge to use: the way the "on/off"/deadman on the tiller worked meant my hands and wrists became really sore because of how it had to be actively held "on". This contrasted significantly with the ECV from Buena Vista, which had a different tiller and on/off/deadman design. That change in design made all the difference for *me* -- no more sore hands !

MUCH easier to use because of that. The design difference meant being able to hold the thing in a different way, and that different way happened to not be painful -- the deadman still works, it just is designed differently (IIRC the difference is basically a push / pull + where it is situated issue).
The second part of that trip we rented from Buena Vista. My original thought was: I will have been shown how to use it by walker, and this way I get to try both companies. I was pleased we went this route, due to the tiller design issue. When I picked up the
Buena Vista scooter at Beach Club's Bell services, the CM who brought it out to me gave me (unasked) a quick demo of how it worked and made sure I was comfortable with it. I realize this is probably not in their expected job duties, but it was certainly much appreciated. the scooter itself was in excellent condition and we had no problems with it.
Fast forward to our December 2015 trip: I needed the scooter so I could handle
MVMCP. I decided to go with Buena Vista again as (1) the tiller design; and (2) much more convenient to pickup/drop off via Bell services (when I can do it on my schedule), especially since I was only there the one night. We stayed at AoA. No demo when I picked the scooter up, but fortunately I now was a "pro" at using it so no problem for me. The scooter we received had seen better days in the wear and tear department (e.g. the tiller handles were rough-feeling due to wear and nto smooth like they had been with the first rental from BV), but only to the extent of being older and worn, not damaged. The scooter itself did its job just fine, no issues.
SW