Jan is my all time favorite time to go. We have been the last 3 years in Jan and loved it. The temps do vary though. One year we wore shorts and swam everyday. But hte last 2 years it was jeans and jackets in am and late pm and jeans and t shirts in day.
The crowds are very low, except for Martin Luther King Weekend. That Sat through Monday in the parks is crazy. So avoid that weekend and you will have a great time. If you have to go that weekend - use fastpass.
Other than that weekend, all the rides are walk-on or have a 5 min wait. The parks open later and close earlier, but due to the low crowds you get to get everything done in the time it is open. Be sure to check the guide for parade and fireworks before going so you know which days to go to which parks. Due to the light crowds they don't have the night time fireworks/shows every night.
Jan is a very relaxing time to go. Since the parks open later - you actually get to sleep a little late (What! Sleep late on a Disney vacation? Unheard of. No, really, you can and not miss anything since they open later). And with the parks closing earlier (except Epcot which is still open till 9 pm) you get to have real dinners and enjoy more non-park stuff in the evening or just relax in the hotel. You will feel more rested after a trip in Jan than you will other times during the year.
Have fun.