I think that hand warmers are a great item for a hunter. There is really nothing to hate about them, except they are a little hard to get open with cold hands, but the are air activated and do a damb good job of keeping your hands warm. I used Hot Hands. I only used them once just to try them out. I usually just buy myself a good warm pair of gloves. The only thing I'd change about them is the way they open up.

I used a reusable one a few years back that you would microwave at the end of your hunt to "recharge" it. Then when in the field you would pop a metal button and it would heat up. They worked great. I try to use mine with a muff and put my bare hands in it so I can react quick if I see a deer. Just make sure that it isn't too hot and if it is get a cover to put it in.

i use hot hands. ill use them during early gun season if its cold, late gun season, and muzzle loader. they can get a little too warm on bare skin. they can also bunch up and become to thick.

I love em. I use em all the time and use hot hands. I wish they would get hotter. Some only raise the temp by a couple of degrees. I think they make em but I aint sure, but it would be nice to make some in the shape of a shoe sole. Normal ones are uncomfortable in your boots. The shelf lifes can be short as well.

Anyone old enough to remember the "Pocket Stove" style of warmer? They were a latched tin box with a velvet outer covering.Inside the box was a fire retardant material like abestos.They would burn coal pieces, or other solid fuels.
It's been a few seasons since Ive seen one, probally collector items now

I rarely use them in Virginia, but I have in Ohio and they work ok. Some will get too hot and can burn you, so be careful where you put it and make sure you can get to it in a hurry.
As for improvements, probably like the man said, a less slick surface and maybe different heat outputs for different places you may stick one.
Believe it or not, I still have my chrome lighter fluid model in the flannel bag. It works fine also.