Albert Greiner wrote a review of Western Mountaineering Bison Gore WS Sleeping Bag: - 40 Degree Down on January 21, 2013
Familiarity: I've used it once or twice and have initial impressions
This is the warmest and also best constructed sleeping bag I have ever seen or used. Zippers work! Bit heavy as you're going to have with a quality -40 bag, but obviously built to perform and last a good while. Very happy with the bag and surprised how well it can pack if you really take your time. Have not tested the temp rating fully, but at 0 can be almost fully unzipped and still warm.