From Australia, where my winters don't get down below 1 Deg. C (33.8 Deg. F) too often, I can wear this without base layers down to 3 Deg. C (37 Deg. F) sometimes the arms are a little cold but the chest area with high collar, keep the torso and neck warm. Materials all quality and the top drys out nice and quick after the ride too or even after a wash so you can put it back on and get out there again. The fit is snug and cut beautifully, this top is in the style category of cycling clothes as far as I am concerned.
What can I say about this top that won't sound like words from a person that is blinded by this top's qualities. I have 3 of these and during winter they are the top I choose, from my wardrobe, almost every ride. I wear a Large in Pearl and the same in Castelli so while I can appreciate peoples comments on up-sizing Castelli I think it is a very personal thing when it comes to fit, not easy when buying over the web I know.