We were there for two weeks over Christmas and New Year's Eve and it was veary crowded. We spent most of our time at Epcot because it did not seem as crowded as the other parks...I think because of it's size crowds are dispersed better.
We did have a great time though. We used Fast Pass for our favorite attractions and then just soaked up the holiday atmosphere. The best night was Christmas Eve, we went to MGM about an hour before closing and walked through the Osborne Family Lights, very festive walking down New York street with 'snow falling'. We then rode on GMR and we were the only ones in our car. A private showing it was fun because the CM improvised a lot because there were only 3 of us.
It was quite cold that year. We are from Colorado and we wore our heavy winter coats and were still chilly at times. We were glad we decided to take those coats!
If your family is not good with crowds Christmas time may not be the time to go but if you are willing to be patient and know that it will be crowded you'll have a great time. It's a wonderful time to visit.
Vicki