We went around December 11-17th. The up side was that the weather wasn't as hot as it usually is every other time we have gone. Another upside is that the crowds were definitely not evident, as we didn't have a line wait over 15 minutes during the entire stay. Typical waits were in the order of 0-10 minutes.
Downside is that, because the crowd size is down, the parks close earlier. EPCOT was the latest one, and it closed at 9PM, with MGM and MK closing around 8ish. So, we spent most every night in Downtown Disney, and the Adventurer's Club (woo-hoo!).
Would we do it again? Absolutely! Take advantage of the Early Entry mornings, and the resort discounts. Get up early, go to bed early, and you won't notice the shorter hours. If you are a nightowl like me, you may find yourself getting bored after 9.
But, all in all, the cooler temp, non-existent crowds, and the 'Pre-Holiday" festivities were a blast. Kind of like a pre-show before Christmas. Have fun!!