I was there the 1st week of FEB this year and in 2002 (going again in 2007). The crowds were minimal. We only used fastpass one time, on Jungle Cruise. My boys were 2 1/2 year olds (twins) this last trip and we were able to enjoy the parks in the morning, grab the bus back to the hotel for nap after lunch, and go back in the afternoon with no problems. We saw a LOT, too! I think it's the best time to go!
Weather is definitely all over the place. You end up packing more because you don't know whether it will be cold or warm. I usually bring things I can layer, especially with the kids.
OH...and bring rain gear just in case. We had 2 days of rain this last trip, but I had packed 2 umbrella stroller rain covers, along with ponchos that saved our lives! Good thing I packed an extra pair of shoes for each of us, too.