-28C (-18.5F) with a wind chill down to -40C (-40F) here this morning
Certainly rideable, but as mentioned, you need to take this kind of cold seriously.
Ride with a friend and inspect each other's face for frostbite in each and every lift line. Sounds gay, but you'll regret it if you don't since you won't feel it.
Personally, having frostbitten the thigh on my lead leg before, I also make it a point to hit the washroom several times a day, drop trou and look for the telltale signs of waxy, yellowish patches