I've ridden as low as 10 degrees, with windchills making it even colder. My feet are always asleep on rides, but I'm not warm until temperatures reach at least 70, so I guess that's to be expected.
My biggest complaint is that it takes soooo long to get all the necessary clothes on for cold-weather riding. But I have a hate/hate relationship with the trainer, so I suffer without whining too much.
More than temperature, road conditions are the determining factor for us when it comes to deciding whether or not to cycle outdoors. Obviously, we don't take the road bikes out on snow or ice; but more often than not, it's the amount of residual sand, salt, grit, etc., left after the county clears the roads that keeps us indoors.
"If we know where we want to go, then even a stony road is bearable." ~~ Horst Koehler