I would definitely recommend using the bus to get to the Magic Kingdom as the busses let you off near the gate, but if you drive, you'll still have to take the boat or monorail over to the main gate. It'll take you alot less time to get to MK on the bus than driving yourself.
For the other parks, it depends. We usually drive ourselves to the other parks as we are usually early birds and get to park near the front. Then, at the end of the day, we just have a short walk to our car and we're on the road without having to wait for the bus. My DH is rather impatient waiting for busses so we like to take the car.
I would highly recommend driving yourself to DTD as it's a long trip by bus usually, due to the numerous stops.
I've never stayed at Pop Century, but have stayed numerous times at AS Movies or Music. I hear the transportation from Pop Century is pretty good, but do expect lines at the most popular times-early morning to the parks, and later leaving the parks after fireworks/closing.
