I have to say I was skeptical when buying this base layer. I wanted something that would keep me cool in the hot months, and having tried a version made by Castelli, and not getting much cooling, I really wasn't sure it would work. My baseline for "cool" is a lightweight summer jersey with no base layer. On my first ride it was 95 degrees and I felt slightly warmer than w/o the base at the beginning, and during the ride I couldn't tell if I was cooler. However, on my way home in the car, I could suddenly feel cool air blowing up my back from the vented car seats. I had never felt that before with any of my jerseys and I had a thought, I wonder if the cooling works better when the Cool Mesh is damp, as it would get during a ride. On my next ride I wet the Cool Mesh, rung it out and put it on and light jersey over it. Ho-ly crap! It was like the AC was suddenly turned as low as it would go!! This really works! Since then this is how I use it for every HOT weather ride and it definitely feels cooler.
The fit is a little big. Normally I wear a large t-shirt, I got a medium in this base layer and it fits well.
Get one, you will not regret it!