We have had two long trips, one in December 2010 and one in September 2011. Both times we had a rental car, but also used Disney transportation sometimes.
Many times the transportation was nothing great or horrible.
One time in December it was just awful for DH and my aunt. They left DHS early, and decided to take the bus. We left awhile later, and drove the car. We left AFTER they did. We were home for well over an hour before they even *got on* a bus. I should mention that they were FIRST in line for OKW from DHS. There were just NO buses being sent to pick them up.
DH had found a number for transportation somewhere, and called them several times. They told him that their way of dispatching had just changed, and they were sending buses out based on how many people were waiting. My aunt has walking and standing problems, and was thinking of falling to the ground, since the transportation people were basically saying that the stops were monitored remotely. She wondered what would happen if she had some sort of spell while in line.
People who were with them in line, who had left in good time to get to OKW for a reservation at Olivia's, not only missed their ADR time, but got to OKW after Olivia's had TOTALLY closed. And it wasn't even an OKW bus that picked them up; an SSR driver finally took pity on them and let them on. Dropped them off after finishing SSR.
That was NOT good. (and please don't ask why we didn't hop in the car and drive to rescue them...no one thought of it at all at the time)
Another time DS and I were trying to get to Epcot from AKL, and it was like CanadianGuy's experience watching bus after bus for *elsewhere* go by, but never seeing YOUR bus. We weren't trying to hit EMH at all, but were trying to get there close to opening, and we missed it.
Now, our experiences while driving were always...GOOD. We have even had tremendous luck parking at MK, either getting there early enough to be close enough so we didn't have to take the tram, or getting there late and just after a huge storm had passed so there was no one to direct us, and we got to the front of the lot again.
So...neutral bus trips, BAD bus trips, and GOOD car trips? We're renting the car again.
Plus, we like the freedom. We like dining at Sweet Tomatoes, we like going to the store, to Costco. To the bank. To Universal if we get the urge for some Butterbeer (we have APs there)! Freedom is good.