We went two Christmases ago. Arrived 12/20 and departed the 26th. I honestly didn't feel like it was too crowded to enjoy. However, I could definately feel/see the difference in the crowds the 25th and the day we left. I think the week between Christmas and New Years is one of their hightest crowd level seasons (if not, the highest), but the week before is not so bad.
It is truly magical and I'm so glad we experienced it, but I don't feel the need to go this time of year again. The weather for the most part was perfect for this time of year. Clear, sunny, warm in the afternoons and pretty chilly in the mornings and evenings, especially once the sun went down. We had early CP show and the venue got quite chilly as the sun was setting because it's right off the lake. Take a heavy jacket if you do this event.
As a family, we toured the park pretty easily and go to see/do just about everything we wanted with minimal waits. (Go early in am to get the most accomplished). Also, make any and all PS's you think you might need. You will turned away at all major sit-down restaurants without a PS. For those who didn't have a PS, they were p**s'ed.
Have fun.