Even though this vest has no insulation, its wind-protective front provides excellent deflection of chilling air. I have worn this over a single short-sleeved summer jersey while using cold-weather arm warmers in the low forties. This was in anticipation of the temperature warming to mid-fifties and higher. Yes, I started out on the chilly side, but after a few miles, I was warmed up enough to no longer be uncomfortable. When the temps reached into the sixties, I easily stowed it in one of my jersey pockets. One of the best features is it has dual-zipper capability for excellent ventilation when it warms up. It's expensive for a vest at full price, but more affordable when Castelli runs a sale. Castelli garments run small, so I ordered two sizes up, 2XL. I'm 6'0" and 185. It fits nicely with no flapping around unless I unzip it significantly.
This is my second one. To me it is an essential piece when the weather gets cold. I wear it over a a regular jersey in the 50s and 60s, and an insulated jersey in the 30s and 40s. It breathes well but is totally wind proof and does a great job of keeping the chill off of your core.
This vest works much better than a jacket as I feel comfortable leaving it on into the 60's although can also take it off if it gets too warm as it is much easier to pack in a jersey rear pocket