Going right around that time, it is going to be busy no doubt, but it isn't packed by any means either. If you want to enjoy the holiday happenings and deal with less crowds, I recommend going about the week before Christmas (sometime around the 15th). The crowds are light and you still enjoy the holiday happenings. The only negative aspect is the parks close earlier than they would around the 28th.
The reason why I recommend this is because in 2001 I was there around Dec 14-15, and had a great time, the parks were nearly empty...however I think that was just the post-9/11 times also. I was just there this new years and what a difference. The crowds were massive, and it was such a big difference from the year before.
Just my opinion:wave: