After reading reviews for the medium weight version, it seemed like the lightweight might be a better choice for me in north Alabama. It turns out to have been a great decision.
I followed the chart for sizing, and am very happy with the way the crew fits. Plenty long enough for my 38" arms, and a lot of waist to stay tucked in.
In the upper 20s, which is a cold spell here, it served as a great underlayer while I hiked in the cold predawn to a photo shoot. That, a lightweight flannel shirt and a windbreaker, and my upper body stayed warm, even a little hot at times. Perfect!
After several washings, I notice no shrinkage. Plus, like all SmartWool, it feels great. I'm a believer!