I haven't seen those before, and fortunately, live in an area where I don't have to worry about temperatures that low.
However, I happened recently out of personal curiosity to go digging for information on what batteries NASA used on their Mars rovers, which also encounter very low temperatures. The cathode and anode chemistry is pretty typical (ICR, I think), but they had a different electrolyte than normally used in order to handle lower temperatures.
One of NASA's rover has been using these batteries daily for roughly 14 years, so even early generation low temperature cells can apparently be quite reliable if well-made and properly cared for.
It looks like Nitecore has some basic performance data on their site, which is very good to have:
http://flashlight.nitecore.com/BATTERIES/18650/NL1829LTHP/
They say ~90% capacity at 0 Celsius. I think that's a bit better than regular lithium ion cells do at 0 C, but the real performance loss happens below 0 Celsius. The performance table indicates they're ok to use at 1C discharge at -20 Celsius, and can expect around 83% nominal capacity, so you're probably ok up to around 1000 lumens at that temperature. That's good. A Sanyo
NCR18650B datasheet indicates around 65% capacity at that temperature and discharge rate, and really significant voltage sag. The Sanyo might actually do worse at lower discharge rates because it appears to recover due to self-heating in the graph I'm looking at, and of course, that's as cold as the Sanyo can go.
At -40 Celsius, they seem to suggested a limit of 580 mA (0.2 C), which would be about 200 lumens from a typical flashlight.
I don't see anything said about temperature for recharging, so I assume your should warm them up to at least 0 Celsius before recharging, like normal lithium-ions
These seem like a reasonable option if you have to store any of your cells in places where the temperature might go below the typical -20 celsius rating of normal lithium-ion batteries.