Got to use the hat in upstate NY in a log cabin with fireplace heat. My wife wore one before the three room cabin really warmed up and it was below 0 degF outside, and sometimes even afterwards. I wore one the first night but it was too warm after that.
We found them quite warm and they seem eminently suitable for sleeping under our 2P quilt when backpacking in cooler weather.
Also they worked quite well for below zero degF middle-of-the-night outhouse runs over a balaclava hat. Odd that I could go out with a warm hat and puffy jacket, down booties, but just boxers and bare legs in sub-zero degF weather with no problem (as long as there was no wind). Of course at that temperature the air is very dry.