I've been in both January and February. I believe, with the exception of holidays and the marathon, it's still the cheapest time to go. It used to be the least crowded time to go, but the last couple of trips I took in Jan/Feb didn't have the low crowds I was used to. At all. I haven't been in Jan/Feb for a couple of years and from what I gather, reading the DISboard, the crowd levels have only picked up.
For me, the biggest con of going at that time of year is how many rides are closed for refurbishment. Two years in a row of not being able to ride KRR because it was being refurbished pushed our usual Jan/Feb trip into March.
The weather is completely unpredictable. It can be 40 degrees and raining or low 70s and sunny. I've had both kinds of experiences there, sometimes during the same trip.
I noticed that with the new tiered ticket pricing, January is the least expensive. That could factor into your decision. I'm pretty sure that January also has the least expensive room rates onsite.
One thing to consider, if you're in the Northeast, for example, is the weather. In the last 5 years we've had 2 blizzards in the NY area delay our scheduled trip--once by 4 days and once by about 36 hours. The 36-hour delayed trip was at the beginning of March. But this is just something to factor in, depending on where you're traveling from.
Something else to consider is whether you'd like to use a pool or go to a water park. Often in Jan/Feb one or both of the water parks are closed, and even though all the pools at WDW resorts are heated, the air when you get out of the pool isn't heated. Nevertheless, I see people using the pools almost no matter what the weather. It's a personal-tolerance issue. This can be an issue with get-wet rides as well (the ones that aren't closed for refurbishment), since it's not exactly pleasant to walk around the MK soaked after riding SM when it's 45 degrees outside!
Also, unless you're in the marathon, avoid the marathon weekend, MLK weekend, and the Presidents Day holidays.