I used Hotronics for years during my coaching/work on the race hill days. If you have to be out in any kind of weather, they are the best. I got the long cords so I was able to hook the batteries to the waistband of my pants. This kept the batteries warms so they lasted longer. Also eliminated the problem of batteries getting whacked by things down around your boot tops....chairs, drills, gates, kids, etc.
Since those days, I'm able to pick and choose better, so when I get cold I just come in. The Hotronics now gathering dust in my closer. If anyone wants a few sets (probably have four or five of them), I'll sent them to you (batteries, chargers, heating elements) for the cost of postage. If it's cold and I have to ski, I use my Garmont Adrenalins with Thermoflex liners. Always warm in those.