We have gone during the last week of January every year for the past five years, and this year was a nice year. Keep in mind the tempeature CAN and WILL fluctuate. One year we were down there and the temps barely reached into the mid 50's, with it going down to almost freezing at night. That's abnormal but it can happen. From what I've seen the highs are usually in the upper 60's, lower 70's. The worst part of this is you never know what you are going to get so you have to pack for everything.
Beside that, we love going in January. Its a lot warmer there then it is in good ol' Kansas City, MO, and the crowds are light. I was really suprised this year with all the rides that were down for refurbishment. Usually there is maybe one major ride down, like Big Thunder Mountain or something, but this year it was I believe three (Splash Mountain, Test Track and Buzz Lightyear). I honestly wouldn't expect so many rides to be down at once like this January.
Another of my favorite things about January is the rates, this year we payed $49 a night for Pop Century, which was dirt cheap, with AP discount. I have NEVER paid rack rates for January.
Jennifer