We were at WDW Jan. 12-15 this year. The weather for the 9 days before we arrived was GORGEOUS. Warm, sunny, fabulous. Our first day was beautiful, no crowds at MK, just PERFECT. Day 2 was MGM, still nice weather, a tiny sprinkle of rain during the parade and another just before Fantasmic. Friday was the day from HELL. It rained about 3 inches. It rained more that day than the total MONTHLY rainfall is supposed to be. It rained the whole entire day, from morning 'til about 8 PM and by the time it stopped we were completely drained from being rained on all day. Saturday it was cold and misty. And no matter which direction we went at Epcot that mist was blowing right in our faces. It was unpleasant enough that we left before Illuminations, even though we really wanted to see it.
The weather in January was a huge factor in motivating us to go again in October. We really only enjoyed half of our days, and half of the parks, because we were short on time. We want to go back and have more than one day for each park in case the weather is yucky part of the time. The parks were not crowded at all, but those last two days were the pits for weather.
I would go back in January if our schedule worked out. Like I said, the people who were coming back as we were leaving had amazing weather - it is just luck of the draw. The weather stayed crummy for a few days after we left, too, so you just never know.