The JL intensive fabric used in Specialized's jerseys is a stealthy performer. It feels ever so thin between your fingers.
However thanks to its tightly woven microfibres it has more chance to draw sweat away from the skin, is more durable than you'd suspect and is highly elastic (as in, it returns to its shape readily). On a cool day it was surprising to feel the jersey keeping me warmer, and little wind passed through the body. Most of the cooling is achieved through the long 10.5in zip, and the fact that it's sleeveless.
Fit is form-hugging and gives aerodynamic lines with a relaxed hem that stays smooth down to the hips. For £40 it would be good to see a bit more detailed finish including reflectors and a covered zipper end inside - it seems to irritate sensitive tummy skin. There's also a single zippered pocket for keys or a mobile.