My friend and I just went Jan 21-25. Had an amazing time. The downside was that is was cold cold cold!! Our first day, it was barely 50 degrees. And that night for fireworks it was in the 30's with wind. The best part was crowds. I wouldn't say they were low, but they weren't bad at all. I say it ranged around 4-6, maybe 7 on our AK day which was a Friday. On our Epcot day when it rained basically all afternoon, the park emptied out. Also, a few weeks after we had returned I read so many threads on horrific crowds that came in February, so I think we may have hit a sweet spot with the crowds.
If you go in late January (early January around the Holidays, the hours should be longer), just know that the wether can vary, rides will be down for maintenance, and the hours will be short. We knew that going in, and I think that helped our expectations some. You also just have to make the short park hours work for you. For example, we would start early, and leave around 5 or 6. With the park closing at 8 or 9, obviously we wouldn't be able to return. So instead, we would leave, go back to the hotel, and hang around the resort and eat and get drinks. So the day doesn't have to stop once you leave. On our last day, we went to Disney springs, which stays open late.
*Other things to look out for: marathons, cheer competitions. January you're pretty safe from random breaks since school is just in, but you never know sometimes. *