I bought three of these to wear to the indoor rock wall gym that I am a member at.  The gym is a converted warehouse with minimal air conditioning and little circulation.  There is often a 10+ degree difference between the air at the top of the wall and the bottom. It certainly gets interesting during the Florida summer. :)

These shirts are amazingly good at staying dry, considering the circumstances.  And they're so light! They really do feel great, and they don't get in my way when I'm climbing.

The bad news, which has been mentioned before, is the tendency for the threads to pull.  It's far easier to do than on a normal shirt. If I run my hands down the fabric, I can hear and feel my callouses catching on it. The sensation drives me nuts (akin to fingernails on a chalkboard for me). Doing this in itself isn't enough to actually displace threads, but the first time I wore one, I saw three or four small places where the threads had been pulled when I took it off.

To be fair, it hasn't happened since. But it's obvious that it can happen pretty easily. So I took a star off for that.

On the other hand, if you (like me) are not doing anything regularly that would cause this, the shirts are definitely worth the money for the comfort.  It's just that they may not be worth the money if you're wearing them for full-contact football. :-)