I was a fair-weather cyclist for my entire life, up until the winter of 2011-12. During the summer of 2011, I found that riding helped me cope with the most emotionally painful event that I have ever experienced, the death of the person I loved most. Also, I lost lots of weight in that summer, more than 60 pounds. In an effort to maintain my mental strength and to keep of the weight, I just continued riding through the winter for the first time. So this will be my third winter of riding.
I do it; but I can't say that I like it. Up until the winter of 2011-12, not only had I never ridden in the winter, I had never been outside so much in the winter! Even as a kid, I barely went outside much in the winter. I ride my bike in winter it for the benefits I mentioned: mental strength to cope with emotional pain, and staying in good physical shape. Rather than go to a gym after work (which I am sure that I would skimp on eventually), I have turned my commute time into workout time.
But -- and I can't emphasise this enough -- it SUCKS! It is a huge contrast from the summer, when I can't wait to get out there. I rode 1000 miles this past July when it was gloriously hot; but I'll ride only about 300-400 miles during a winter month, almost all in commuting.
I just layer on the clothing. Mike's comment about wearing only a sweatshirt and shorts during days in the 40s made me cold just reading it. This morning it was 40 degrees when I left home; I had eight layers on my chest, including three sweatshirts. And I had two pairs of gloves and three scarves. (I use a scarf around my neck at all times if the temperature is under 90 degrees; I really don't like the wind on my neck.)
My legs are fine in the cold; I can comfortably wear shorts down to about freezing. In weather colder than that I turn to sweatpants.
As the weather gets worse, I'll mix in a small lined jacket and then another large thick sweatshirt amongst those layers. And I have a fourth scarf and additional pairs of gloves that I can use. I also have a big lined sweatsuit that I can wear over everything else in extreme cold. My personal record is 11 degrees.
So I'd say that I'm a fair-weather cyclist by nature, who has become a year-round cyclist by necessity.