I got one back in 2006 and ditched it in a dumpster after two nights!

(1) By morning, I was so tangled up in it that it took me several minutes to get out of my sleeping bag!

(2) Once I ditched the thing, I didn't notice that I was any cooler, under comparable conditiions (rainy and upper 30s/low 40s F.

IMHO, save your money. If you're down to marginal temps for your sleeping bag, that's what your insulating clothing is for.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey