I commute all year round in most weather here in Colorado. I manage to combine bike gear with some of my ski gear to be nice and toasty, though this year I also need to pick up a pair of windbreak tights and a better pair of shoe covers.
If you're dressed right, it's really no problem. FWIW, I use a Surly Cross-Check with 700x28 Armadillos in the summer and 700x32 knobby tires in the winter. Haven't had reason yet to need studded tires - snow melts pretty quickly down here, so not much ice.
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