We were there this year for Christmas and New year for 17 nights, while it was busy we didn't find it uncomfortably so. It's just fab at Christmas, so much to do and just not enough time to get to it all. Magic Kingdom was closed at 10.30 on Christmas morning due to full capacity, we had no intention of going there that day anyway, had decided on Epcott which is huge and much better able for large crowds...we had a fab Christmas day there.
I wouldn't rule out Magic Kingdom entirely though, it so just so amazing at Christmas, the parades, decorations, shows, fireworks... we went just for these, didn't do any rides that we had done in Paris and still managed to go twice to MK in Orlando.
At times the traffic was bad between Christmas and New Year, and it was almost impossible to get a table at a restaurant some nights but proper planning would sort that. There are the only downsides I can think of but no way would I let these stop me, it was by far the best Christmas we have ever had, highly recommend it