I was there at 8am on Christmas Day and left around 7pm. The first hour or two was GREAT and then it got EXTREMELY crowded. BUT, we knew it was going to be that way going in and were prepared for the crowds. It was manageable, but don't go intending to do a "regular" day in getting all your rides done, etc. We did that the Monday before. Christmas Day we did what we could and then spent the rest of the day checking out special Chrismtas shows, etc.
We went back to MK the 26th around 6:30 to see Spectro and Wishes. The MK was open until midnight this night and the crowds never really let up when we left around 11:30pm.