Tested in temperatures of minus 10-15 in US winter, while photographing the great outdoors.
Anticipating temperatures anywhere between minus 5 and minus 30, I knew I needed more than double gloves to keep my fingers working, and chemical handwarmers seemed the best option. I looked at reviews of various brands of chemical handwarmer before deciding that Mycoal had more satisfied customers, and less people complaining the chemicals wouldn't warm up at all, than other brands reviewed.
I was with a group of photographers, all with a supply of 'tea-bags', and was slightly worried when one person complained hers would not warm up at all: was I fated to have the same experience?
However, after vigorous squeezing and shaking my handwarmers began to heat up, got really toasty and stayed hot all day, in my pocket or in my gloves. This was my experience throughout the trip. All the handwarmers I used worked properly, there were no dud packets. While we did not get the lowest temperatures possible for the place we visited I would be confident in using these handwarmers in extreme cold.
I would definitely buy this brand again and would recommend them to anyone.