In reply to Mr Eddie: Yep, not saying that they're a bad idea, just the way the gear thing is phrased it makes it sound like you get in this bag and suddenly you're only losing 10% of your body heat over any given period. It's not quite that simple, or effective.
These bags are miles miles better than space blankets which (being blankets rather than bags) are not so useful if it's at all windy.