More than you can imagine!
Seriously, the decorations and special shows and parades are wonderful. At the same time, lines for the most popular attractions can be 2+ hours. Restaurant reservations need to made ahead for anything more that just fast food.
If you get to WDW often, it's certainly a treat to see the parks and hotels in their holiday decorations.
But, if this is a once-in-a-long-while visit - I'd choose another time!