Mainly use the disposable chemical ones for deer hunting. They work well.
Purchased a rechargeable one for elk hunting this last fall. Brand new, charged it before I left. After a 1200 mile car ride, 6 mile hike into the mountains, and a couple thousand feet gained in elevation, I pulled it out one afternoon to charge a device while I was taking a mid-day break. It was dead; and the charging port terminal was bent. Not salvageable. Failed as a handwarmer, failed as a battery charger. I would not recommend as your only source of heat or power.