I only carry one of these products when day snowshoeing in the mountains. If I had to call for a rescue, at least it's something, in addition to whatever insulation is on me and in the day pack.
The one I carry is an envelope Adventure Medical Kits calls an emergency bivy, (all their blankets say 'SOL' on the package, although some are very different in composition). It is a breathable, water resistant, nonwoven gray polyester with a reflective coating on the outside of the envelope with Velcro closures, and rolls up into a 8"x4" stuffsack weighing just under a half pound. It is very different from the lighter polyethylene blankets also sold with the SOL label, and by Heatsheets, and totally different from the crinkly, reflective Mylar space blankets.
I'm thinking that if on deep snow, there would be plenty of airspace between me and the ground. The polyester is much easier to handle than the polyethylene and mylar materials, and if very cold and hypothermic, handling stuff can be a major problem.
Hope I never have to use it. Hope 'SOL' doesn't mean what I think it might.