I use Neos all the time, usually while I am coaching cross country ski races. They allow for almost any shoe or boot to slip inside and instantly keep your feet warm and dry as if it were 30-40 degrees warmer. They are ideal if you go from being very active and warm in your regular footwear to not for a while or vice versa. I met a guy who trekked across Greenland in a pair with running shoes inside.
