Never thought of chemical warmers as something to rely on -- they're just a luxury alternative to a heating pad. Nice at a Redskins game. I keep some in my car in the winter mostly for acquaintances on winter outings who don't have good gloves.
Experience tracks others -- ones I've had last a couple to a few years past the expiration date.
For reliable warmth I put much more faith in the down booties, mittens/gloves, Smartwool socks/Lowa boots/Gore-Tex gaiters/Marmot pants, sleeping bag, etc. warm and waterproof clothes that I keep in the car fall-winter-early spring.
And matches, firesteel, blah, blah.
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Funny you should mention that... I always end up giving mine away to someone who DIDN'T bring a proper coat or gloves.. I hardly ever use them myself these days.
I have a whole new appreciation for good socks. I have a several pair of Wigwam in a couple different systles and I don't think I'll ever go back to those white Hanes socks...
Waterproof pants.. I have a super lightweight set and a heavier set for colder days... Which reminds me.. I need to get the kiddo's out to the Tubing hills!