I wear this in temps ranging from the low 50s to the low 30s under a wind/waterproof uninsulated jacket and I couldn't be more comfortable. It allows me to heat up my core without over-heating. Good deal.
Keeps my hands warms down to low 40s. When it was damp and the wind chill started to dip into the mid-30s, though, my finger tips started to chill. They are comfortable, though, and they breath pretty well, too. Over all, a good value with good construction.
I live in hot and muggy South Florida and ride about 100 miles every weekend. I always save the Castelli jersey for my longest rides as it breathes very well, fits comfortably, and just makes me feel faster. In 90 degree heat with 70% humidity, this actually keeps me pretty dry. The sizing advice that you see about Castelli is spot on... XL for them means L for most American riders. (I am 6'1 and about 205 and find that the XL jersey fits almost perfectly, whereas with PI, L is the right size.)
Having been scared away from Castelli due to the high prices in the past, I was thrilled to be able to get my hands on this jersey on sale -- it really lives up the hype. Looking forward to my first Castelli bib purchase.