We just spent 8 days at the World and used WDW Transport a lot. My mother was with us the first few days so we drove then, and then switched to buses for the last few days. I would say that by time you get through the parking gate lines, get parked out in the north-forty, wait for a tram, and get trammed-in you could have waited for a bus at FW. I found the buses to be very frequent and doubt I ever waited more than 10 minutes. I would just as soon not hassle with remembering where the car is, dealing with idiots who are too busy reading signs to drive in one lane, etc. I'm here to relax though, so take it for what it's worth.
I would have liked to have a car when going resort-to-resort though... that can take a lot of time. We ate at Citricos on our last night, and the quickest route I could think of was boat to MK and then boat to GF. We were late. Not too bad, but the line for boats to GF was stacked and backed with folks leaving the MK and trying to avoid the CRAZY line for monorail service. We had used a car to get to GF a few nights prior when we ate at Narcoosee's with mom and used valet parking. Ten dollars! We could have taken a cab! You would think that if you're dropping $250 for dinner they would give free valet! Ah, Disney! Anyway, I hope some of this is useful to you.