We went in January 2004 for DS's 8th birthday. NO LINES ANYWHERE (literally). However, although we were *physically* prepared for cold weather (we brought sweatshirts, jeans, gloves, ear muffs), we were not *mentally* prepared for the cold weather!! When we arrived, it was in the mid-70's and very nice. Shorts were fine. Then the sun went down and we were COLD!! The next day we made sure to dress for the weather. Water rides were COMPLETELY out. In the morning it was in the 30's - 40's and we wore all our cold weather gear. By the middle of the day, we were stripping off sweatshirts and storing them in a locker. By evening, we were running to the locker to get the clothes back on!!
I LOVE January for the non-existent crowds, I didn't like wearing ear muffs in Florida. We may have been there during an unusually cool snap, but I don't think I want to do it again!!!
If the cool weather wouldn't bother you, it's a fabulous time to go!!