We have never gone at Christmas but we have been to WDW 11 times and never rented a car. We got everywhere we needed to go with very little trouble using the buses. Since we only go to the parks and Downtown Disney we never felt the need to rent a car. Plus Magical Express is free so you don't have to pick up your luggage at the airport.
I would think that parking in the lots and just plain driving that time of year would be more trouble than it is worth. It is the most crowded time of the year from what I've read. We have been staying at the POP and am going again next September. Can't come soon enough.