Tried them today for the first time. Performance is about on par with a Hot Hands pack. A Hot Hands pack, however, lasts longer. I followed the directions for the Zippo hand warmer, and it lasted about 5 hours (6 hours advertised), so it worked fine for my morning hunt. When it went out, it went out completely. A Hot Hands pack tends to slowly burn down, and can still be warm 10 hours or so after you activate it. That said, Hot Hands are a bit expensive at almost a dollar a piece if bought in bulk, and the Zippo hand warmer uses a very small amount of lighter fluid to burn for five hours, so the Zippo may be more economical in the long run -- especially if you go through Hot Hands packets like we do in my hunting party!