Been to Disney 2 times at Christmas, once as a kid and once for our Honeymoon. It is a great expereince, the extra hours, the special shows, decorations etc.....just be prepared to work around the crowds. I do not think it is a big deal since although the park is busy, Disney has more staff and is running at capacity.
Last time we went on December 26th and didn't venture into the parks until the 27th. Invest in Tourguide Mike or Touring plans or somehting and the crowds are workable.