my personal rule is if it's lower than 10 degrees, i don't ride, but i don't have to ride. it just makes getting from class to class more enjoyable and saves me from having to bum a ride off someone if i have to go somewhere off campus.
I have yet to ride in less than 24F. 17 is the coldest we've had but I wasn't going to get up at 5.30am, especially to go riding in it. Where I live, there just aren't the same extreme temperatures. I would probably never skip riding just because it was too cold outside.
rode at 0°F (-16°F windchill); everything worked, but it was no longer pleasant. and my speed dropped dramatically (from 14-20mph to 6-12mph). i decided that was cold enough, and worked from home today.
for me, i can ride comfortably down to about 15°F, before i get to the why-bother scenario. if i had artic-level gear i might feel otherwise. my bike didn't seem to care one way or the other.
-6 to 25 degrees for a month now on my 6 mile morning commute, plus every variety of snow and ice you can imagine. But it has been fine anyway, and well worth it. As the temperatures dropped through fall I pretty well laid out what I needed to wear and got accustomed to the freezing weather. Only when it gets below 10 degrees I have to ride slower to keep the wind chill factor in a manageable range, but mostly I just dress according to the temperature and go as usual.