Just put this to the test on a 90-degree day. While I initially shared Timothy M.'s skepticism about the fabric in the shoulders, arms, and down the center of the back, I appreciated the comfort on the bike. I really did not notice how high the temperature was until I stopped riding. This jersey helps moisture disappear. The perforated fabric used in the rest of the jersey would not provide much sun protection on the shoulders and arms--sun protection is the reason for the more-solid fabric there. The jersey eventually felt a little sticky across the shoulders after the ride, but never felt uncomfortable. Sun protection overall seems quite adequate.

I first ordered a medium, which was uncomfortably small across the chest and waist, and felt too short. (Have a look sat the length in the video; video doesn't lie.) Replaced it with a large, which fits looser overall but still snug across the waist, so it stays in place. And it's long enough that I can get into the rear pockets easily enough. If you don't want a race fit, order a size larger than you usually wear.

This is advertised as a summer weight jersey, but it uses fabric across the shoulders and the top of the back that's not breathable at all, and there's also a wide strip of the same fabric running down the center of the back.
The jersey is also very short, especially at the waist in front. I'm only 5' 10" and a large is too short in front.