The weight and performance of this garment are stellar. First time out rode 1.5 hours in misty on-again / off-again drizzle and core stayed dry and comfortable. Have also worn it in cool, dry conditions as a wind vest and it seemed reasonably breathable...but that is a difficult judgment to make. As advertised it is uber light and easily packable.
The single flaw, and one that makes me hesitant to wear it -- it rides up in the back. Yes, there is elastic at the bottom of the rear panel but that is inadequate to keep the tail low. Once it rides up, loose fabric at the shoulders flaps annoyingly in headwinds or at speeds above 21 mph. I have even sewn a strip of silicone-backed elastic at the rear hem but that did not solve the problem.
Disclaimer ... I am 5'9" tall at 150 lbs, with a 39" chest. I am right on the border between Calstelli's M and L sizes, and the vest I'm struggling with is a L. It fits reasonably snugly but not tight. Perhaps my experience would be different if I could wear a medium without feeling constricted.