I think this one is individual choice. You see conflicting opinions regarding the Disney bus service. I think the bus service is excellent, but also generally still prefer to drive. If speed in getting there is the primary concern I think your car will almost always be quicker. The only exception would be if you arrive at the bus stop exactly when the bus does - and you are at the last stop on the route. You also do have the "tram" time from the parking lot, but I don't think the "wait" for a tram is ever very long - contrary to krec's comment my experience is that any wait for a tram is certainly on average much shorter than the wait at the bus stops. Personally if you don't mind driving I would take the car except for maybe to the MK, where the buses bypass the TTC. Of course, riding the monorail or ferry from the TTC to MK can be fun also - much more so than riding the bus. (I don't get disney magic from riding a disney bus, it is really just like any other bus ride).
As far as pick-up points, most of the time you will take a tram from the park exit to your car which eliminates the walk, depending upon how close you park to the tram drop off. I have had my family wait at the tram drop-off point while I went and got the car if the kids are tired - and you don't want to carry them. Also, I have parked at the Boardwalk and exited EPCOT through the international gateway after illuminations. That gives you a chance to enjoy the Boardwalk area either on the way in or the way out and I then had my family wait at the entrance to the Boardwalk Inn while I went to get the car.
Finally, I echo the point about taking the boat from PO-Riverside to Downtown Disney. Not the quickest way to get there but it is a nice boat ride for the family.
They also have boats that just go back and forth from PO-Riverside to PO-French Quarter. We did that just to check out the FQ "dragon" pool. (Don't bother - the Riverside pool and slide is actually much better).
