we went on Christmas day maybe 5 years ago and it was so busy the parks reached capacity by noon, but it was a lot of fun! If you're smart about fast passes (and maybe consider paying for max pass) you can still get a lot done
We were there last Christmas. When we left in the afternoon to go to the hotel and relax for a while it was at capacity or at least they weren't letting anyone in the gates. We asked a CM as we were leaving if they thought it was open back up later. They said they expected it to open up in an hour or so. As we were walking back to harbor the line for Disneyland reached past the middle of the esplanade. When we came back a few hours later for a dinner reservation, it was back open and hardly any line.
We are going to be in the parks on Christmas Eve and Christmas day and expect it to be off the charts busy!!! We've purchased Maxpass (after humming and hawwing) so we shall see how much we get done but first and foremost we are going to see it all done up for Christmas and enjoy the atmosphere.
Same here we have been before and yes it is busy but for me its not unbearable until about after they do the lighting on the castle and there is no way to walk down main street. Other than that i do ok with the crowds and love being there during the holidays