I've used them on winter rides and hiking, and found them helpful. I also placed them between my socks and booties on top of my feet, though if I used my shoe covers I ditched the booties and just had the chemical warmers between socks and shoes. I concur that shoe covers really help a lot.

Maillotpois, thanks for the tip on back heat patches! If my knee & neck allows I want to do some winter hiking this year and I didn't know they HAD back heat patches, good to know!