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.