Just got one of these bad boys. I love the weight. Enough to wear by itself in moderate weather or during high output periods. Awesome base layer for super cold. Stretch and cut is great (a little lower in the back). Seems to breathe well. Gets stinky quick (for me).
This is my standard alpine rock climbing shirt. I fell in love with it the first time I wore it. We were on Cathedral Peak in Tuolumne and the temp was in the 70s at base. Higher up there was a cold wind and I couldn't believe how dry and warm I stayed. It was the only shirt I had on. I unzipped the collar while climbing and zipped it up at the belay. Highly recommended.
For as light and compact as this shirt is, it's ability to block the wind is incredible. I overheat very quickly in this shirt in moderate temps. I've read reviews on other sites where several folks have complained about discomfort and irritation at the seams, but I have not experienced this.