I own several of these shirts in a variety of colors. I reach for one of these almost every time I run. They fit well, a bit relaxed without being square or frumpy-looking. I'm 5'1", 106 lbs and have these in the extra small (XS). They are a mesh fabric all over, and made of polyester, so I don't overheat in them and they wick sweat very well. They're very lightweight, I've worn them in 80 degree weather and felt fine. The only negative about these shirts is that they tend to snag easily and when they do, look a little like pantyhose with runs. If I'm only running, they do fine, but if I'm doing a race and heading out to eat afterward or anything like that, I'm not sure they'll hold up well. I recently ordered the Brooks Versatile EZ T which is NOT mesh to see if they would hold up better (FYI those are sized smaller, so if you need an XS in this Asics shirt, you'll probably want a S in the Brooks).

All that said, I do recommend this as a comfortable, relatively inexpensive shirt. If you can find them for $15 or less, stock up!