We were there the last week in January last year. A few days of amazing weather, a few days of crumby weather. We didn't mind the foggy, wet, cool days and nights, the only drawback to that was that the night we stayed for wishes was so foggy we couldn't see most of the fireworks.
Overall crowd level was shockingly low, especially a few of the days at MK. We did rope drop and did pretty much all of fantasy/new fantasyland, including multiple visits to princess fairytale hall and Peter Pan and Pooh before 11am. It was almost creepy how deserted the hub was on a few afternoons. Most rides were walk on, even in the afternoon. We used FP+, booked 60 days out, but with few exceptions (TT, ETwB, TSMM) FP+ was not necessary. We were able to get dinner reservations at normal dinner times for every place that we wanted to go (including CRT and BOG). The whole trip felt relaxing and low stress.
There were a few downsides that could effect people differently-- shorter hours (didn't matter much to us, we have young children so staying super later was never really in the cards), several closed rides and food areas (would have liked to go on splash mountain), we didn't use the pool ( but saw some who did). We saw tour groups, but that was about the limit of our interaction with them. Nighttime entertainment is more limited, but that that just meant we had to be a little more careful with our touring plans to make sure we saw what we wanted to see.
Overall, the low crowds more than made up for any negatives. We're planning on going again at the same time in 2015