JPLwrote a review of Castelli Leggero Vest on July 1, 2012
Best vest I've owned so far. I'm 5'9" with a 39" chest and the L is a perfect fit: very snug but not constrictive. I hate it when vests flap in the wind so I wanted the "race" fit. Once it's on over my jersey I hardly even know it's there. It's even fairly snug (but not tight) around the neck, so there's far less wind that sneaks in through up top. And as others have mentioned, it rolls up incredibly small.
One other note on the fit: the arm holes are elastic and somewhat tight. Which I like; again, one less spot for the wind to sneak in. I've noticed that if you're standing it'll feel like the bottom of the arm holes are riding up into your armpits, and I was definitely nervous when I tried this vest on the first time thinking this was really going to bother me. However, once in the saddle in the crouch position I've experience no discomfort whatsoever.