I had the original vest and it is a perfect layer with a long sleeve jersey for shoulder season mountain biking. I never overheat or find myself chilled by the wind, and having some extra pockets is great. I got the new version for the two-way zipper, which further enhances the temperature control. I also wanted a bright orange color for visibility on road rides. I wear this thing all the time!
I’m usually a large in t-shirts and jerseys, sometime a medium (6ft 170 lbs). The large fits perfectly trim. It’s a little short, more of a roadie cut, but it doesn’t mother me.