We were there 12/16 - 23, and I was shocked at how manageable it was. Later in the afternoon it picked up in the parks, like it does on any day...I have a trip report in my signature if you're interested.
Our schedule: 12/16, arrival, checked in at boardwalk
12/17 - morning EMH at AK, left at 11, Sanaa lunch at noon, DHS evening for Jingle Bell Jingle Bam dessert party (FPs here: star tours, toy story, tower)
12/18 - resort hopping, DH and one kid to Epcot (it was busy, but it was a party night at MK) while I was with the other kid who had been to urgent care for ear infection that morning while he napped, afternoon/evening MK for
MVMCP (which was awesome as far as crowds go, in my opinion)
12/19 - Epcot morning and early afternoon, AK for a couple of hours in the evening (FPs here: safari and everest)
12/20 - MK morning (super easy morning! left around 2ish), Disney Springs evening - crowded!!
12/21 - Epcot all day, not terribly busy, but we were mostly in World Showcase and not riding rides. Had FPs for Character Spot (didn't use), Spaceship Earth, and Frozen, plus candlelight processional package w/5pm show
12/22 - DVC tour, MK morning with "free" FPs from the DVC thing...crowds still seemed ok, no worse than July or August crowds we've encountered, left at 2pm (last MVMCP night). DHS evening after dinner with FPs for first Fantasmic and Tower.
12/23 - departure day
I carefully planned our days, and that may have helped for crowds...but I was pleasantly surprised by how manageable it was. It felt no worse than summer, and in some ways, less crowded.