Hi Freddy
Christmas is a great time to go, the only difference i noticed is no one says "Happy Christmas" its all "Happy Holidays" in Orlando.
We were there from 10 December - 3 January and I think it started to get busy around 20th and to be honest it was horrid from then on, the ride times were far too long to attempt to get on anything, we even attempted a visit to Busch Gardens but most rides had 3 hour long queues.
If you are staying on site, take advantage of the Extra Magic Hours, this is when we rode most of the rides. We did this for Magic Kingdom on Boxing Day and we got on nearly everything we wanted to by about 10am, its definately worth it. The buses run that early so conveient (sp?) too!!
I would definatley go back at Christmas time as its a very special time the decorations are beautiful and you even get snow on main street.
You will have a fantastic time!!
Lisa