A couple of years ago, I used to winter commute (45 minutes each way) but for a variety of reasons, don't anymore. My cut off was -15 celsius or black ice, whichever came first. There's nothing quite like a slipout on black ice to bring your heart rate up.
Regardless, the secret is studded tires. I would switch over to them when winter hits and as long as there's no black ice, the traction is excellent.
I loved commuting in the winter, provided the amount of snow on the roads wasn't crazy (I had to commute through serious downtown traffic so I was more concerned about the drivers than I was about my ability to be in control).