We were there for New Years, it was crazy! Have never seen a place so extremely crowded, I do not know if it is the same for Christmas, but as you do a lot of people love to be there for the same reasons as you do, decorations and the magic.
There were a lot of people getting frustrated, impossible to get fast passes, they were all gone by 10:00am, lines would take literally hours, I mean 6 hours, the non popular rides were ok. Since I noticed a lot of people fighting and tempers blown, we decided to just be there, and do what was possible but not to get upset or try to do the popular ones, we can always go back at another day... then we were able to enjoy it.
Plan in advance your restaurant reservations, if you can find any, I think is a little late, but even if its not your first restaurant choice try to get reservations so you avoid long lines even for food.
Good luck, I would loooove to go for Christmas! But after our New Years experience, there is no way I will ever convince my husband!!!