I've used mine down to 18* F by warming the canister before use--putting the icy (!) canister in my armpit under my insulating jacket and doing a vigorous (!) "keep warm" dance for 10-15 minutes. You also need to keep the canister warm while the stove is burning, because the process of fuel vaporization chills the canister. (Remember "latent heat of vaporization" from high school physics?) Although officially not recommended, I use a windscreen that goes 3/4 of the way around and keep feeling the canister to make sure it isn't overheating. An exploding canister could ruin your day! A pan of warm water in which to place the canister is good, too, except that you somehow have to heat it up first. Although clunky, you may want to put the canister in the foot of your sleeping bag overnight along with your camera and water filter. Warming the canister prevents the more volatile propane from coming out first, leaving a very sluggish canister with nothing but isobutane.

If the temp is consistently below 20*F, you probably want the kind of stove that uses an inverted canister, such as the MSR Windpro II or (if you can find one) Kovea Spider. This configuration turns the canister into a liquid feed, but no pumping required. These have a generator tube which warms and vaporizes the fuel just before it gets to the burner. These inverted canister stoves, I've read, are good down to about zero F. Colder than that, you want liquid fuel. I've decided to invest in the MSR Windpro II not only for cold weather camping but also for group camping since it is much more stable with larger pots than the standard stove that screws into the top of the canister.