I've tried using those emergency blankets in a few situations:
*Elk hunting in Colorado Rockies..... snow storm came in unexpectedly (as it does out there in the Fall).. I was 2 miles from base camp, but a rugged walk considering the storm. Since I had everything I needed for a night over in the woods, I set up camp, and wrapped the mylar around me.
*Winter Wilderness Survival Camping with the Scouts.... Built a small shelter next to a blow down, and crawled in for the night, wrapping the mylar around me.
In both instances, I found that the mylar doesn't breathe at all. And, the increasing amount of moisture in my clothes, especially during the deep cold, totally OFFSET any benefit that the reflected heat gave.
Bottom line: Those blankets are OK for short term temporary use, like while transporting some injured person to the hospital. Or, perhaps wrapping someone who has been in icy water when there are no other blankets around.
But, they are NOT good for general sleeping use in long term survival situations.