My wife uses the Hotronics, and likes them a lot. Prior to them she had chronic cold feet.
The version that I bought her does not have a complete foot bed, but rather just an oval element that is adhered to the existing foot bed just behind the toes, with an adhesive backed material that you then cover the entire foot bed with. They are fairly easy to install, but will add to the hassle of removing the foot bed and possibly the liner, so make sure all fitting is done before they're put in.
Basically, you locate where you need the elements placed and make a slit for the wire in the foot bed. The wire is thin and flat, and I ran it under the foot bed - but off to the side-to the back of the heal and put another slit in the liner along the bottom seem just towards the outside so I could run it up the back of the liner between the ankle bone and the tendon to reduce the likely hood of a pressure point. To avoid movement and wear on the wire between the liner and shell I covered it with duct tape (no job is complete without a piece of duct tape). The battery packs were then mounted to the shell on the back towards the outside edge of the boot - they are not a hindrance here. They unplug and pop off of a small bracket for charging - the bracket stays on the boot ( think these brackets were an extra option that had to be purchased separately). The battery packs have a clip similar to a spring loaded belt clip that attaches to this bracket ...supposedly this can be used to slip directly onto the boot shell, but I did not feel it was secure enough and opted for the more positive mounting bracket. This installation allows me to easily remove the liner from the boot, but makes it a hassle to get the foot bed out of it, but I felt the extra hassle was warranted as the other option was to run the wire inside between the liner and her leg, and I feel this would definitely end up with a pressure point and a problem trying to keep the wire in a 'comfortable' location. It took a little time experimenting with the location of the wire under the foot bed, but we were able to eventually route it so she could not feel it under foot ....again, use the duct tape and experiment.
The only problems we've had is that the batteries will not hold a charge for an extended period of time ie we tried charging at home before packing and flying out west to ski and they gave up before the morning was over - charge the night before for the next days use. This year the adhesive on the foot bed cover is starting to give up and can result in a wrinkle ...haven't focused on finding a solution to that just yet ....this usually results when she removes her foot, and for now I can smooth it by hand before she puts the boot back on.
There are several different temperature settings on the battery pack. Everyone is probably different, but she has had the most luck by putting on a warm dry boot in the morning and turning them on low and leaving them there.