I had a pretty favorable experience with these at some late fall campouts with my son's Boy Scout Troop. The first one, the temps dropped down to 30 or below, which I know is not very cold for late fall/early winter camping, but for my son and I, who aren't big on cold weather camping, the handwarmers we had put in our sleeping bags sure helped get us through the night, so much so that I bought the bigger hand and body warmers in a 3-pack for the next campout. Those along with the addition of fleece sleeping bag liners really made a huge difference at that campout. As these have expiration dates, they are probably another example of something you should check before you leave the comfort of your home. As others mentioned, maybe these aren't something you should rely on for emergencies, but based on my recent experience I'll continue tossing a few in the pack for those colder nights. If weight and space permit, tossing in more than you expect to use for yourself and others could also be a backup for the time that one goes bad or doesn't work quite like it is supposed to.