In reply to felixthelion: Not been at Xmas/New Year, but we did have a great holiday there a few years ago in early March. We were skiing at Mammoth in temps of about -15, and cragging in the Owens River Gorge in shorts in temps of about 25 in the sun!
I think it depends what you're going for - cragging or bouldering. A lot of the bouldering is fairly exposed, and if there's any wind it could be miserable. If sunny and still it could be great. The cragging in the Owens is very sheltered and on both sides of the Gorge so lots of potential to follow the sun. I think most other cragging in the area is a lot more exposed.
Another thing is where you might be thinking of staying. We've generally camped, although on the holiday mentioned above we were in a B&B in Mammoth. Camping at that time of year would be cold and long hours of darkness. I think you could probably get a cheap apartment in Bishop, but we've never checked that out.
It's a great place, and we keep going back!
Hope that's some help.
Cheers
Andy
Cheers