We were there this past December, and I can tell you one thing: I will NEVER go back in the summer!!!
Here was our schedule:
Dec. 11th Epcot--almost totally empty. Walked onto every ride except Test Track, had to wait 10 min. Kids got to meet Stanley, Nemo, and other characters...there were usually only a few kids ahead of us to meet characters.
Dec. 12th Animal Kingdom--again, almost empty. Walked onto everything, no waits at all, not even on the safari. Even found last minute spots on the parade route, no problem! Met all the Pooh characters in mere minutes--not long lines at all.
Dec. 13th and 14th Magic Kingdom--more crowded than past two parks, felt very crowded to us, but we were used to walking on rides! Didn't wait on any lines before 2pm. After 2, we used FastPass a few times (rides in Fantasy Land were 20 min+ which seemed like a long time, but compared to summer, that's nothing!) Character meet and greets were also great--no long lines, except for Minnie--maybe 7-10 minutes!
Another benefit to us are cool temperatures--no one is cranky, sweaty, or thirsty. Walking is easy when it's not 100 degrees out.
Two things that were not so great: parks close early, but that was not an issue for us as we have little kids, 2 and 4, who are in bed by 7:30 anyway. Also, the weather was cold enough (high of 70) that we couldn't use the pool at the Grand Floridian, where we stayed...but we'd gladly trade these things for the low crowds!!!