My major issue with them is that they are not water permeable. If you sleep with your head under the covers, you end up with all your breath condensing on the side _you're_ on, and it drips on you, your bedclothes, whatever. You breathe out a _lot_ of water vapor.

On the other hand, they work. I've camped when it turned out my sleeping bag wasn't warm enough, and the space blanket made the difference between being able to stay warm or not being warm and having to leave. I wouldn't want the space blanket to be my sole source of warmth, but it works.

Phil