I have only ridden down to the high 20's/low 30's Fahrenheit and have not been able to enjoy it as much as I would like. I am still working on keeping the feet warm. However I rode last evening at 42 degrees and really enjoyed it. I was dressed perfectly so I did not get cold or sweat either. This was after riding in 77 degree weather a couple of evenings ago.
Prior to this year I would not have dreamed of riding in weather that required more than normal summer cycling clothes. Because I rode in the cold, and at night for the first time also, I was able to stay connected with a group that I started riding with last summer. Putting up with the cold was worth it to be able to stay connected to my group and not have to ride the trainer. I would encourage more cyclists to try it. If you meet a person of the opposite sex that you like, you can be certain that if they ride in the cold they are a real cycling enthusiast!