I was there last December from Dec 11th to Dec 23rd.
The weather is fantastic and so are the crowds (very few long lines, just for those star attactions like Soarin and Everest). The crowds before the 20th were smaller and as it got closer to Xmas it started to be a little more crowded, but I believe that between Xmas and New Years is the most crowded week.
I suggest going from Jan 1st to Jan 8th as you'll get much lighter crowds (I hear that LOTS of people leave WDW on the 2nd) and you'll still see the holiday decorations (I believe they are removed on the night between Jan 6th and 7th so you can see the parks with and without the decorations)