We went last year (Dec 13-21st) and although we enjoyed our trip, we probably won't go at that time of year again. The weather was actually in the 70's the whole week which was nice, but didn't feel like the holidays. Our days were shorter compared to previous trips due to it getting dark earlier and the parks closing earlier as well. I also felt it was pretty crowded the majority of our trip. I think the secret is out that December is a low crowd time
That being said, if you haven't been to Disney when the holiday decorations are out, it is worth doing at least once. Definately check out the decorated Disney resorts (Grand Floridian was my favorite!) We also really enjoyed the Osbourne Lights. My advice would be to go as early in the month as possible. HTH!