In 2010 it has been REALLY cold ( like 30 degrees) just before we arrived. But overall we had temps in the 60 - 70's.
There were 3 days in 2009 that were quite cold...probably areound 40 degrees---but we managed. I'd rather be cold than hot so it did not bother me too badly.
We love January for the low crowds. Martin Luther king Day and the Sunday preceeding it can be a little more corwded, but, again, overall in my opinion, it is not bad at all....especially with smart use of Fast Passes.