Well, comfort aside, I reckon the idea with these things is to keep you alive more than anything else. I guess long as it keeps you dry in a rainstorm and keeps the wind off your back, you got a better chance than without it. If the reflective heat claim is at all realistic, that'd be an improvement over garbage bags enough to warrant the added cost I suppose.
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)