I tend to stay in resorts closest to the parks I want to visit (I don't always visit all parks on the same trip), so I find myself on boats more than buses. I do tend to drive more if I'm traveling with my Mom, because she has trouble standing for very long (I usually rent her a wheelchair for the parks as well) so after a day in the parks, it's better for her if we're driving. We don't stay in the parks as long when she's visiting for the same reasons. Out of all the times I've driven to Epcot, I've only had trouble getting out of the parking lot once, and that was because there was a big accident on the exit road, so no one could leave the lot. We sat in the car about two hours that night. But otherwise, it's been very easy.
In my experience at MGM, they seem to start parking in the morning in one of the back lots instead of up front, so you have nearly an equal wait for a tram or a bus (that's why I like the Epcot resorts - gotta love those boats!) If MGM always started the parking up close in the morning, I might be more inclined to drive there. As it is, using Disney transport works better for me.
I almost always drive to Animal Kingdom, but I don't visit that park as often as the others. Sometimes my allergies act up too much to enjoy it. I rarely drive to the MK, because you have to use transport to get to the park anyway. I might as well already be on a boat or monorail or bus, as drive, take the tram, and then have to take a boat or monorail anyway.