I spent a night in the woods (on purpose, not a real emergency) sleeping with an emergency blanket. It was not at all comfortable, but the blanket seriously worked. I would wake up whenever it moved to stop covering me because it would instantly become cold, then feel much warmer as soon as the blanket is back in place.
The reflective action is impressive. They also act as a wind break and turn whatever clothes you are wearing into a more effective measure against convective heat loss.
The poor thermal insulation and condensation were predictable. I hadn't predicted the shifting wind which chose to balloon my orange cocoon with cold air every 20 mins no matter which orientation I chose to lie in. I finished the night with a bungie cord around the sack, under my armpits. That of course left my head, shoulders and arms exposed but kept the wind out. Not recommended.
After this experience I upgraded to reflective mylar survival bags. NB "bag". These are a little harder to find than survival blankets but if a bag can be so badly affected by moderate wind, a blanket would be so much worse.
The reflective action is impressive. They also act as a wind break and turn whatever clothes you are wearing into a more effective measure against convective heat loss.