I am a huge proponent of spaceblanket and have used them on numberous occasions. I have a similar one to what Jerry has as well as the disposable ones (I like the silver/gold combo for easy use).

I have made shelters and lined them with the space blankets and feel they definitely helped warm me up.

I keep a couple in my truck, along with a poncho liner and cheap sleeping bag. I also keep a couple in my "go" bag as well as my backpack for camping that I often refer to as a "rescue pack."