I too use Hotronics. Here is my experience with them. They work but... You have to find the right setting. For a few very cold days (-35 wind chill) my feet were still getting cold with them. So I kept increasing the level and still cold feet. Well increasing the level was the wrong thing to do for me. My feet got too hot and started to sweat. Finally, I tried setting them at level 1 and only occasionally through out the day go up to 2 for a bit and then back to 1.
That made all the difference. No more cold feet.
The other thing, as people have mentioned, depending on where they are installed the chair might hit them. Also you might break the cable if you take your boots off with the cable still attached to the batteries. Where the cable meets the plug is pretty vulnerable to breakage. So what I've done with mine is duct tape around that area to give it a bit more strength and I always unplug them before I take my boots off.