We just left yesterday, and if this week is anything like last week, you are going to be fine! We got in Tuesday, and went to the park about 3:30 Tuesday afternoon. The crowds were GREAT!! We walked right on to just about everything we wanted to, including Matterhorn. The weather was great too, cool, but not COLD. A light fleece and we were fine. Wednesday was slightly busier than Tuesday, but still GREAT crowds. We rode Haunted Mansion at 9:30 with no wait whatsoever, same for Big Thunder Mountain, and Santa's Reindeer Round Up (all between 9:30-11 am). The longest wait we saw Wednesday for Star Tours was 30 minutes, compared to 85 when we were there in July. Toy Story Midway Mania ALWAYS had a wait, no matter which day or what time, but it was a quick 20 minutes at about 11:00 Wednesday morning. Thursday was busier than Wednesday, but again still really do-able. The only times you really felt the crowds were at night during the castle lighting/snow (which was truly AMAZING!). Then, the throngs of people on Main Street were difficult to get through, but once you made your way into another "land" things thinned out, and were great. The only day that was really "busy" was Friday, and really it wasn't too bad until night. We went back after nap, around 6:00 pm, and it was NUTS. Long waits for everything, and people everywhere. But we still had fun! Overall I thought it was PERFECT. Midweek was all but dead, in my opinion (especially compared to last time we went, in mid-July). In general, evenings (after 5) were busier than daytime. My advice would be to ride the big holiday attractions (like It's a Small World Holiday, Haunted Mansion Holiday, and Santa's Reindeer Round-Up) when you get there in the AM, then you can kind of putter around for the rest of the day. We were there 3 days and one evening (4 day PH), and by the last day, we felt like we had really done everything we wanted, and many things more than once. I thought December was GREAT, and we're planning on going again next year.
ETA: The weather was FABULOUS! weather.com said 65 or 66, but our car registered between 70 and 73 each day we were there. It was BEAUTIFUL and I was HOT in a light sweater! Nights were cool immediately after the sun went down at 5, and got "cold-ish" by 7 or 8, but a fleece was enough for the adults. We did put our kids in winter coats most nights just to keep them from getting sick, but still it was really great weather.