Negatives: Some socks the warmers have so violently stuck for giving me 4 or 5 times in succession broken when removing torn. Everywhere the black dusty content was verteilt- also guaranteed not healthy. So carefully remove, socks it works great. Even negative: the relatively high price for a prolonged use.
Nevertheless: Warm feet on cold days are heavenly. Therefore, 5 stars!
I got me a week after testing the 40 package ordered, the winter can be like even colder, my feet stay warm :)
