I injured my knee last year and decided against surgery and to take the long road of therapy and time. On this last trip I rented an
ECV a couple of days when my knee just couldn't take it anymore.
I don't drive a car because of an anxiety issue and had a lot of concern over the ECV (getting behind the wheel so to speak) but it is very easy to drive, easy to get off and on, and most lines are easy to manouver (but I recommend a little bit of practice doing sharp turns in an uncrowded area since you need to be able to do that for many ride lines).
I also found people to be VERY HELPFUL when I was trying to do stuff that proved a little stuggling.
For instance, at the food and wine festival, at first I would park the
scooter, get off, get the food samples, put them in the basket and then drive over to where my mom and dad had a table. Eventually I just found it easier to go through the line on the ECV - the CM's were extremely helpful and even the other guests.
Disney makes it very easy to be in a wheelchair or ECV.
And since you do have some mobility, if there is anywhere you don't feel comfortable driving it the ECV, park it, take your key and walk a little (which I recommend anyways, you don't realize it but you can really stiffen up sitting on that thing - every 45 minutes, I'd get up and walk for a couple of minutes or my knee would lock).