I feel I have to defend this time of year a little bit. I visited during Christmas 2000, leaving on Jan 1. The place was definately very busy, but with all the preparation I did and the knowledge I had from my previous visits, we didn't wait in any line over 25 minutes. That's not to say that there weren't lines over 25 minutes, we just didn't wait in them. We saw everything we wanted to with the exception of the Indiana Jones Stunt Spectacular (bad timing on our part) and Test Track (the lines were either hours long or the ride was down). We used Fastpass to the extreme, went to every Early Entry day we could, and realized that we would never see everything. There were not any children with us, so we did skip most of Fantasyland in MK and caught only 1 parade.
I don't want all of these comments to scare you, because you can have a great time going over Christmas. You just need to prepare a little more.