The windstopper fabric is pretty amazing. It's almost as light as any stuffable flimsy rain cape I've had, but it feels more durable and is totally windproof and decently water resistant. I was a little disappointed that it's not stretchy enough around the shoulders so at higher speeds it starts to flap. But that annoyance turned to being pretty chuffed when the rain started: I got stuck in a total downpour first time wearing it and the vest totally did it's job. It stuffs down pretty small too so it's a good one to choose on those days you're not sure about the weather.
Summary: great fabric & quality, light as a fart, annoying flapping, fits great in a jersey pocket no problem, slightly longer than my other Castelli vest (good), totally rocks in the rain. Deducting 1 star for the flapping.