DH and I went the first week of January two years ago. It was excellent!! The lines were almost non-existant, and we almost thought Epcot was closed since we were the only people in some lines. The weather was great - low 70s during the day, and low 60's at night. Only once did I want a sweatshirt (We're from NH, so our definition of warm might be a bit different)
I would definitely go in January again
Jen