Some time in February, I think 2009, I had the marvellous idea of going outside to shoot at -27C. Did not have a car, went biking for about 4km and back home. At the time, I had my old EOS 10D and sigma lenses 17-35 2.8-4 and sigma 105mm. Besides 50mm 1.8 II.
After 3 hours shooting with no problem, the LCD started to react much slower and the AF i the lenses to be very slow. I decided to leave home, that was the first time that my toes got frozen, very painful to get them back to life
This year in March we were outside photographing the aurora and it was -20C. Canon EOS 5D mark II, 24mm 1.4L II, Sigma 12-24mm and Canon 24-70mm 2.8 L.
Besides that we were freezing, toes again as well as hands, no problems at all, 4 hours outside in total. Batteries get empty quite faster than normally, but I did not miss any shot because of this.
I usually put my equipment back in my Lowepro old minitrekker and fastpack with lots of silica gels (I always carry them inside the bags) and I have not had any problem. Just be careful and leave the camera to warm up inside the backpack.
Next time, toewarmers for sure