We went the last weekend in January in both 2011 and in 2013. This particular weekend both years happened be a long semester break weekend for my kiddos, which is why we chose to go then. Both times we experienced low crowds, smaller lines at restaurants, and reasonable wait times for rides and attractions. In 2011 it was cold (we're from Michigan, so it was fine with us) like in the 50s and we needed layers and couldn't swim. In 2013, it was in the 80s and perfect every day. So, January is sort of a gamble for weather, but both times we enjoyed very manageable crowds and wait times.
I hope that helps!! Whenever you go, have a magical trip!!!