I prefer a tight fit and as a regular Gym user, i bought a large, but large is pretty damn big, so i wish i got a medium. That said, it doesn't really matter, the quality is excellent as is the colour and i'm really happy with it.
Its a good vest but made out of something closer to heavy cotton rather than Nikes usual poly tops. Very comfy though and good for climbing. If you are after something tight i would go a couple of sizes down as it is fairly baggy. All round great top.
I've been looking for a long running vest for ages. I'm only 6'2", but apparently large vests are designed for short fat guys, so tend to show my belly button to folk on the street. This, however, is faultless. It's a great fabric, no chafing (although there are seams) & covers my dignity! It's so good, I just ordered a second one :)