been there the last week in jan twice (2014, 2015) and am planning on going back this jan.
as stated above, weather can be highly variable-- one year most days were cloudy/rainy with highs barely reaching 55, we wore scarfs and gloves. The other year it was glorious, rained fo maybe an hour out of the 10 days we were there and was in the mid-70s eveyday. People were in the pools, but it's not really pool weatherm either way.
crowds-- also variable. Yes, there were tour groups, but for the most part they really weren't an issue, i think this was especially true because we were pretty careful about only going to recommended parks. Most tour groups are from south america (esp. Brazil) and i wonder with major downturns in the economy there if there will be fewer groups this year. some days seem randomly busy, some days (especially is weather is bad) the park can be a ghost town. Some lines are long, most aren't. (having a touring plan helps immensely). Overall crowds seemed on the smaller side and definitely manageable.
other things to think about-- price is just about the lowest it is all year (second half of jan/first half of feb) More rides tend to be down for repairs than at other times of the year (ex: splash mountain is generally closed for the entire month of jan). Parks close earlier on many days (ie. MK 8pm), Night time entertainment may not occur on all nights, or may occur only once per night (F! once per night, MSEP may only occur a few times a week). This year, NFL Probowl will take place on 1/29 in orlando, with some events taking place in WDW resort over the week before the 1/29, including activities at ESPN and a parade at MK. At this point, specific dates of these events and the impact on crowds is unknown.
overall we think the tradeoff of avoiding 100 degree + weather and super long lines is worth the negatives.