They do stink quite a bit but if a deer is able to smell that then he' s going to smell you anyway.
I have tried them and they work ok if you keep them in your pocket and just want to keep your hands warm but they suck if you put them inside your jacket or anything. They need too much oxygen to stay going and don' t stay lit inside your clothes.
Your better off with a few of those disposable heat packets tucked into your shirt pockets to keep your body warm.