We were just there 21st thru 26th. Here's how we managed:
Sunday we went to MGM after lunch and only did a couple of attractions. We had Fantasmic package that evening and dinner was at 4pm. We watched the parade and had dinner imm. after. Then Fantasmic at 7pm or so. So we didn't get much done, but we got Fantasmic out of the way.
Monday we went to MK mid morning and only did a couple of rides, did Buzz twice using Fastpass. We had lunch ressies at noon at Liberty Tree Tavern. Finished lunch just in time for the afternoon parade. Went back to the room for a quick swim and nap.
Tuesday we to to MK at opening and managed to get all rides done by around 1pm. We didn't do any of the mountains, though.
We returned to the room for a break and went to Epcot for PS's at Le Cellier. Then we took monorail to MK for Spectro and Wishes.
Thru Tuesday it felt crowded, but not unbearable or too stressful. Kids did well and we felt our pace was fast, but not too hurried.
However...By Wednes (24th) we could tell the crowds were getting thicker and thicker. We went back to Epcot for a little while and then had CP that evening with dinner at Rose and Crown. Didn't see much that day, though.
Thursday we got to Epcot early and spent the entire day there. We got FP for Test track and Mission Space early. There were soon gone by early afternoon. Crowds were heavy and I was glad we had finished MK on Tuesday. I can only imagine how they were that day at MK. Oh, even though it was crowded, we had very little waits for everything we did all week. Using FP for the popular rides is the key to finishing those attractions quick.
Friday we left for home so I can't comment. However, check out did not seem crowded.
Also, you MUST get PS's as early as possible. Every sit down we did there was someone who had just walked up wanting a table without a PS and was told none were available - period! We never waited more than 10 minutes for a table for any of our PS's.
I enjoyed my Christmas trip and have lovely memories of it. My kids had a blast. And I was expecting worse crowds. But I don't feel I need to go at Holiday again. It's out of my system now.
Good luck.