We absolutely love January, but we don't mind the cool weather when we're there. This year, our first couple of days 1/20-22, it was downright freezing! We were wearing our winter coats and the wind made it feel unbearably cold. By the 27th, it was in the 80's and we were too hot!
It's the hardest time of year to pack for, but if you can think in layers you will do fine. Crowds are usually low, but prepare to park hop so you can avoid the giant Brazilian tour groups. Be flexible, brings different styles of clothes and you will enjoy January.