Crowds should be low that time in December. I was there last December, first weekend and it was busy. It was also the Candle light processional and Marie Osmond was unveiling her newest doll (at the time it was her newest). That being said, I love the decorations. Small World Holiday was a huge hit with my 3 year old and my 20 month old nephew. Saturday the parks were crowded and we used fast pass and single rider lines to ride most of the big rides. Single rider was very nice. Since my sister and I had the kids we couldn't spend a lot of time in lines and being able to get right on w/ single rider helped a bunch (it is only on Indy and Splash at DL)
Nightmare before Christmas will take over the Haunted Mansion and it was very fun. They had a lot of different themed foods. Great deserts. Our weather was warm during the day and cool at night. My sister from Vegas thought it was cool all day, but since my daugher and I left the snow, we though So. Cal was warm.
Have a fun planning your trip and have even more fun on it.