My observations based on personal use of this item:

1. They are wind-proof.
2. They are good at reflecting heat.
3. They suck in blocking conductive heat transfer (i.e. butt-to-cold ground).
4. Whenever you move, they sound like you are wadding up a patato chip bag.

So, from a heat transfer standpoint, their performance is adequate in blocking 2 of the 3 heat transfer modes (radiation and convection) and are inadequate in conduction.

I still carry them, but you have to have something else to put between you and the cold, hard ground.

Your mileage may vary.

Jim