We were there the 13th to the 24th this year and I thought the crowds were very managable. We are early risers, so we get most of our rides and such done in the morning to early afternoon. The only thing we needed fast pass for was Toy Story Mania. I walked on Space Mountain, Test Track, Soarin was maybe a 20 min wait. By the weekend before Christmas (Sat was the 20th I think) we saw the crowds start building. But we had planned for this and did mostly non-park activities on the 21st -24th (Pirate Cruise for the kids, carriage ride at Ft Wilderness, checked out resort decorations, bead activities at AKL, DTD, that sort of thing).
I would go at that time of the year again. Can't beat the weather! We are gong in August this year and I am not looking forward to sweating to death.