I bought an HotGear boot bag (with also shoulder straps, backpack like) last year,
Very nice to put your feet in a warm and dry boot every morning but must pay attention the heat level one selects (if left on the whole night).
On one occasion we drove 4 hrs with the bag connected to the car 12v all the time.
Heat is not selectable in that setup (only when connected to the home 220v, can one select three warmth level, the highest of which it is suggested not to let on for long)
afte having it run full power the whole night.
When reaching our destination, boots were as soft as overcooked noodles, I could even feel the sole almost bending underneath my feet while walking. Luckily I had used my old and soon to be retired plug race boot (xt17). Apart from that, once got used to the fine points of using it, fantastic product.
Not only boots, but also gloves and helmet can stay dry and warm...not only when plugged in.
We discovered that the HG bag keeps the warmth for at least a couple of hours (depending on variables like external temps, car temps and so on).