Hmm, mixed feelings about this.

There may be some value to this in generally mild climates, where you wouldn't normally expect to get cold. So in the short term, maybe it's better than nothing.

Moisture retention is the big consideration. Really, this is a space blanket with a plastic bag draped over top. If you wear this over your clothing, you will quickly become damp and then wet from normal insensible perspiration. And then you will really be cold. So if you wear it, put it close to the skin, and live with the clammy feeling. If you're actively walking, it won't ventilate, and the moisture build up will happen even faster.

Besides that, I don't like the "disposable" aspect. And what's the shelf live in a package? Better off, I think, with the reflective bubble wrap that Susan suggested. That is true survival stuff.

My vote? Not worth the money. Carry a fleece cap and an oversize trash bag instead.