I was there this year from 1/1 thru 1/5. We went to MGM on the 2nd and I haven't been there when it was that crowded in a long time. All I can say is use fastpass. On the other hand, we went to AK on the 4th and it was empty.
Oh and we had one day when we could wear shorts (the 2nd) and then it was very, very, very, very cold. You never know what the weather will be. I always check the weather channel's website every day for about a week before we leave.