Last time we were at WL it was Christmas 09 and the bus service was wonderful. We kept our car parked the whole time. I think it is much easier to leave the park after a long day when your feet are tired and just head over to the busses. Much easier than waiting for a tram and searching for your car.
Also, remember that with staying at WL you will have to go through the main parking ticketing gait and show your hotel parking pass every time you return to your hotel. If you are going during a busy time that will slow you down. We were blissfully unaware of the LOADS of traffic during that week, until we saw the HUGE backup of cars at the parking ticketing gate on the way out of WDW! We were so glad we used the busses then, they just skip all that.
Also, remember that with staying at WL you will have to go through the main parking ticketing gait and show your hotel parking pass every time you return to your hotel. If you are going during a busy time that will slow you down. We were blissfully unaware of the LOADS of traffic during that week, until we saw the HUGE backup of cars at the parking ticketing gate on the way out of WDW! We were so glad we used the busses then, they just skip all that.