We were there 1/21-1/31/05 & were a little disappointed with the crowds. We felt it was much busier than Jan 2000 and our trip in Sept 2004. It seemed like there were many tour groups there - not sure if we had really bad luck & just hit everything at the wrong time, but we saw quite a few long waits for rides. We saw 60 minute waits for Test Track on 3 different days - got a fast pass each day instead. 45 minutes for Thunder Mountain - 2 different days, 35 minutes for Space Mountain - 2 different days. Stitch's new ride was hardly ever less than 30 minutes when we were there - the line wrapped around the building - never seen it like that before. Frequent waits of 40 minutes for Jungle Cruise & long lines at Pirates & the Haunted Mansion. To many peak-time visitors this may not sound bad - but, these definitely aren't the wait times we were used to.
Not sure if we just had bad luck, or if more people are going in Jan to escape the cold & enjoy the low crowds. Between the lines & poor weather, we aren't planning a return trip in Jan anytime soon.