I wanted to wait on this review until I had a change to wear and properly test these out. They have now been used for running, weight lifting and running.
These are the lightest weight shirts I have ever owned. No extra weight or drag but they do cover my body like a shirt should. The thinness means they do not hold very much sweat so unless there is a breeze they will stick to you real fast. While wearing them on a stationary bicycle training session for over an hour they cool off nice as long as you have a good fan. Unlike going shirtless they seem to solve the extreme cold spots where the fan hits most direct. Once dry they provided a breeze blocking action.
The only downside I can find: Almost ZERO UV blocking. Out on a run I burned almost as fast as I do without.