Pinch it and roll it between your fingers, and determine how quilly the down is. Basically, FF s strength is that they are essentially a custom shop. If you need the genuinely warmest bag on the planet, or the lightest, give them a call. Otherwise, get a Marmot or a Western Mountaineering for a VERY close second. Feathered Friends Just bought a Feathered Friends' Hummingbird - added 800 down, draft collar and Dry Loft.
Build quality is excellent.
So far I have been very pleased with the bag. It is rated to 10 degrees F which is just about what I have experienced. The bag IS tight, I find I cannot bring my knees up and when I roll from side-side the bag rolls with me. I am willing to tolerate this because of the packability and weight. In terms of ventilation in the summer I do not know as I have only used the bag late fall and winter when the half zip has not been a problem.