I recently found my old metal lighter fuid powered warmer I used when I was a kid. It still works, although a little stinky. But, if the deer is gonna smell that, its gonna smell you so I think the smell is a mute point. I remember one time the little wick cap came off, and it got upside-down in my pocket. The lighter fluid seeped out, and caused a chemical burn on my leg. Other than that, it really cranks ou the heat. I used it a lot while ice fishing.
The chemical warmers that you can stick to your shirt over your kidneys are awesome. As ghost Dancer mentioned, you can also tuck one of the body warmer sized pouches in your collar. It will make a huge difference. I did just that this weekend, I was starting to geta chill, and tossed my heater in my collar on the back of my neck, and in minutes I was very comfortable and warm. It was 20 degrees out, windy and snowing to beat the band.
For cold weather outings, they are great. Nothing ruins a hunt faster than cold toes.