I only had my M6 out in about - 21-22 C - I did not experience any problems exept that it was cold to hold.
Remember to put it into a very tight plastic bag, when you go inside in the warm house to avoid condensation - and if you have the possibility to let the camera go from -25C slowly to 0C to 10C before inside temp of say 22C then you will not shckc the camera. I am not sure what the english word is but before i enter inside my house i have a "varanda/porch" that is about + 5C in the winther and I always leave my camera there for a while to get i temperated.
Michael, I spent 10 years in Canada's Arctic with temperatures down to -57deg.celsius! I used both a Leicaflex SL2 and R3 with no problems whatsoever. I do not suggest carrying your camera outside your coat for a long time. You do need to keep the battery warm. My main concern was film breaking and static electricity. Thank goodness for manual film advance/rewind. If you're going to be there for an extended period, you may want to have Leica "winterize" your camera.
Collin's advice to have Leica "winterize" your camera in advance of the trip is good advice.
I believe there's an article on a shooter using M-cameras up in the cold wintery north in one of Leica's publications this month. I think it was Leica News...
Lastly, if you're wearing mitts (and I suspect you will be!
), do as Roger suggests... check out the rapidwinder, grip, and soft release combination. With the rapidwinder... you won't have to be trying to get your winding finger in behind the film advnce lever while trying to advance the film. The advance is at the bottom of the camera and it's long enough so that you can advance the film without looking. Once you get use to the wind and shoot... you'll find it quite efficient.