Over my time in the outdoors, I have spent an unplanned, or semi-planned, night outdoors about fifteen or twenty times. On at least three occasions, I had no blanket or sack, but had a stove or the means to build a fire. My bivy sack on most of the other occasions was a nylon sack, coated on one side and uncoated on the other. Any of the AMK products work better than that sack, but the sack was good enough that I was able to get some rest and function better when dawn finally came. Best of all is a gore-tex bivy sack, but that is relatively heavy. As important as the shelter is the source of warmth - fire or stove. There is nothing like a warm drink at 3 in the morning to boost morale.

For personal use a bivy sack is more protective and warmer for the weight, but for treating a casualty, a blanket gives the access that is necessary for adequate treatment.

I now carry the Thermo-Lite 2 Bivy as part of my core gear. I am not sure that I will use it very much, since after years of trying, I have finally figured out how to get home before the sun sets.