Went for a ride yesterday, temps around 35-38F. I had a Polar S210 and Cateye Mity 3 on the h-bars. The LCD update on the polar was in slow motion, and when the HR number was flashing, it never became solid enough to read. If it were much colder, I think the display would have been completely useless. The temp specs are 14F to 122F, so it may be that its battery is a little weak.
The Cat-eye was solid. Numbers were dark, and updated normally. I couldn't find any temp specs for it. We'll see how it does on one of my morning commutes when temps are in the 20s.