Hey Clint,
I use Hotronics in my ski boots, and wouldn't ski without them now. The battery pack has 4 heat settings. I turn them on the lowest setting when I leave the house so they are warmed up when I put them on at the ski hill. Most of the time I use the #2 setting, and they will last for a full day of skiing. Not sure how good they are at -40, cuz I wouldn't be skiing then! I have thought about putting them in my hunting boots. They cost around $200, and I'm on my 4th season with them, but the batteries are getting weaker.
http://www.hotronic.com/products/fw/index_m4.html