We used ME in October with our daughter who uses a wheelchair. They were fabulous. We finally got to the ME counter, or, rather, the line in front of the counter for ME when a CM who was at the entrance for the line stopped me, I was pushing DD10 and DD7 was with me, DH was off to the bathroom or something. I was pleasantly surprised as he brought us around to the front of the HUGE Line, to talk to a CM at the counter to check us in. It appears that not all buses are accessible, but they knew we were there and were able to be sure that the next bus coming for the Contemporary would be accessible. They saved us the wait in line as we might have to wait longer for the bus. The CM said it'd be about 15 minutes. No sooner did I get to the ladies room with DD10, get her out of her chair and onto the potty than I hear DH sending DD7 into the bathroom to get us, the bus was there and ready (they need to board WC users first). I apologize to DD10,ask her if she can wait and we go!
It really was hassle free, no need to worry about our luggage, just the carryon we brought and the small bag/backpack each of us had, plus DD10's things she needed, all of which hung on her chair.
Renting a car might be difficult as you would have to worry about the chair fitting in, transfering to a seat, when on the bus you can just stay in the chair.
The luggage was brought to our room before we were back from dinner that night.
The transportation at Disney was really great too, no real hassles with the chair, however we stayed at the Contemporary for the express reason that we had easy monorail access for our Epcot and MK days. The buses were very easy to use as well.
With so many of you I would save my $$ and just use ME!