Here's some real info and not pointless drivel like the posts above.
January is actually far clearer on average than the main tourist season of June - Sept. Your odds of getting views of Everest are quite high - especially if you plan your itinerary to stay the night at Rongbuk. That way you'll one chance the morning you drive up from Shegar and another the next morning.
Temperatures can be very cold overnight at Rongbuk (-15 / -20) at that time of year and I'd recommend taking a decent sleeping bag with you (can be rented in Lhasa)
A quick google search will give you some weather stats.
As you'll see in some of my other posts - winter is a great time to visit Tibet as long as you're prepared for the cold. I highly recommend it.