I would recommend doing both at least some of the time, but, after my last trip, I will drive most of the time.
I have made several trips and stayed on site; first at All Star Magic in January 2000, Caribbean Beach in January 2012, and Port Orleans Riverside this September. The first two trips I always used transportation and it was fine. Yes we waited for busses, had crowds, and leaving the parks could be a bit of a wait, but I didn't think it was terrible. I did go at low times.
However, this September I rented a
scooter because my wife had foot surgery. I didn't like the idea of getting that thing on the buses and it was easy to take apart and put in our car so I drove. It was fantastic! I find it very hard to believe someone could leave by car before someone headed to a bus stop and NOT get there quicker. A lot quicker! If the buses were faster in a family's experience it was an anomoly. Of course. the buses are relaxing - once you are in and seated - so that is a plus.
Disney has a very well thought out interstate highway system that got us where we wanted to go quickly any time of the day. (We did make a few wrong turns due to a GPS that didn't have updated maps.) The parking lots are huge and there are large muli-lane entry and exit roads. At opening or closing we were in and parked or out and on the road quickly every time! Parking attendants are helpful, trams run consistently, and you often can park close enough to walk (or scoot in my wife's case), if you get there early for opening. There were no issues going from park to park or getting to any reservations. We drove to the TTC and took the monorail to MK and this worked fine.
After this experience, I will probably still use buses, but I will drive more than ride.
P.S. I am sure the bus drivers are as great as everyone else was at Disney, and would be very nice and accomodating for scooters, we just didn't want the hassle.