I've used the M8 at -5 degrees F, albeit only for about 25 minutes. Mechanically, it worked perfectly. The most significant limitation I encountered was battery life, only getting about half of my usual number of shots. This is probably the case for just about any camera though.
My M2 took -28C in its stride during a Czech Christmas a year or ten ago. No batteries to grumble, of course, whereas my M6 TTL demanded new cells for its meter during a Tallinn visit a few years later - a balmy -12C or so on that occasion (shutter seemed to be fine, though).