We used to take the Disney buses to all of the locations. On the last trip, which was in the off season, we drove. Boy was it faster. Parking was not a problem, and AK we actually parked closer than the POR bus stop. We drove because we were hauling a stroller for the 1 year old. When we go next summer he'll be 2.5 yrs, so well use the buses again. In the Summer parking is scarce at Downtown Disney, and distant at AKL.
A lot depends on your resort. POR was fairly quick, I've heard nightmares about some of the delux hotel buses. As you wil read there are no direct buses from one resort to another, you will need to either use your car or taxi unless you want to take over an hour for the trip.