First, do you use handwarmers: Sometimes
and if so how often? Only when it's below about 40 degrees
What do you like about them? Heat source.
What don’t you like about them? Expense, one more thing to carry to the woods.
What is one thing you would change? Can't think of anything.
How long do you need them to last for? 12 hours

I've used the air-activated ones, they don't seem to get very warm, in my book. I use the ones that take liquid fuel, like lighter fluid, but contrary to the claims, they won't keep burning unless I expose them to air now and then, like pulling them out of my pocket and moving them around.