I just got back from a January trip and LOVED it! With the help of tour guide mike, we walked on to most every ride. The longest we waited was for Toy Story Mania and that was only about 20 minutes.
While the first few days were cold (very unusual, record lows), the rest was in the high 60's to low 80's. It didn't even rain once in our 9 days there!
Now, I haven't been any other time, but with my lone experience, I would never hesitate to go back in January. It was a wonderful intro to WDW!!!