My winter bivi sack is more breathable and less water resistant then gore-tex but I only use it in cold weather. We did the Spearhead traverse for three days in the first week of May this year and everything was always completely dry. Like I said before I have never had any problem with condensation but I wouldn't use it in rainy weather.
I've been on Baker many times with others using a Megamid and they usually have a miserable night due to the winds.
When I go high in the winter I only use the bivi and a snow tube - fast and quiet!
