I have been during the holidays at least six times, including 12/19-12/26 in 1999 and 12/27-1/3 in 2001/2002. Depending on when Christmas falls, the week before Christmas is often much less crowded than the week after Christmas but the crowds pick up as you get closer and closer to Christmas. Christmas Day at MK was the most crowded day that I have been at any theme park ever, but it was also very magical and, with planning, very enjoyable. You MUST get PS's if you want a sit down meal (and I would generally recommend that at the holiday time anyway just for sanity sake). You also need to be more flexible as the crowds ebb and flow, trying to take advantage of opportunities that present themselves. And you need to be very patient and willing to wait things out sometimes (we sat for a long time so we had a good seat for the Christmas day parades and fireworks; ditto for the NYE Illuminations at Epcot). I love being at WDW at the holidays and wouldn't discourage you. Frankly, NYE at Epcot was very smooth and no where near as congested as I expected despite the crowds.