Leather won't heat to your body as fast as synthetics, but assuming you're dressed for sub-arctic conditions, you probably don't care about that. I've never heard of a Brooks failing in that environment, nor, for that matter, any other saddle.
Riding them soaking wet from rain has killed more than a few Brooks , though.
Remember that lots of stuff goes wrong at -20F, so what do I really know? Personally, I'd just drive or take a bus.
So far I've been using it to get around campus - unfortunately, while our campus is outfitted with quite the nice bus system (free campus busses), the unfortunate truth is that if I relied on the busses, I'd never get to class on time.
Brooks users please describe your experience.
Do you always use padded shorts over the Brooks or just other shorts or trousers; on commuting and also on long-haul trips?
I've so far used jeans, jeans + long underwear (it be chilly up here in the Wisconsin), and padded shorts (on the trainer). All have resulted in far, FAR less discomfort than any of the plastic/foam saddles I've previously owned.