At -15C there's a chance of your camera shutting down (not forever but till she warms back up). But every camera is different. I was out the other night -10F with my 7D and my buddy with the exact same camera and lens was having all kinds of problems with his lens and camera failing. His 7D kept shutting down and with his lens he lost use of aperture...but mine worked perfect the whole time.
When it's that cold out you really need to keep your camera cold the entire time. So a hand warmer could end up creating condensation and freezing up your camera. When I go out on trip like that I keep my gear in the bag that I used all day and put it in the coldest corner of the tent, cab of a truck, or garage if you have one. If you end up bringing your camera into a warm spot (above freezing) give it at least an hour to warm up and make sure there's no condensation on it before you turn it on.
Cameras are far more durable than people think. As long as it's out of the harsh elements, you're usually just fine.