I have one, the Austrian made Dachstein boiled wool liners that kept me warm (inside shells) at -40 F. in the 1079 Pre-Olympics at Lake Placid. I was just amazed at their performance. I began by wearing US Army wool glove liners with them but soon removed the gloves as they were TOO hot!
My other liners, not tested below -5 F., are double layer thick fleece liners. I think they might be as warm as the Dachsteins. They feel very soft and comfortable and at -5 F. were very warm.

