I found them to be a waste of money. My feet still got cold. I've tried everything for cold feet -- sock liners, hand warmer packets, even bought a pair of the military Mickey Boots -- after a few hours I still had cold feet.
This I have learned:
1) The number one cause of cold feet is a sock/boot combination that is TOO TIGHT! Don't wear the big thick "hunting" socks. I buy good quality (yes, expensive ) wicking wool blend socks that are relatively thin (the Cabasspro catalogs usually list the wall thickness of their socks). Then when on stand make sure that your boots are not laced too tightly -- untie them and pull them apart so that your feet are not gripped tightly. It took me years to figure this out.
2) For most people, sitting motionless for hours in freezing temperatures is going to lead to cold feet no matter what you do. All you can do is hope to minimize it so that you can still concentrate on hunting.