This is the MOST USEFUL piece of clothing I have. It works great as a layer under a shell, but can really we worn at any time. It doesn't pick up any odors for a long long time. But the most important thing is that it is the perfect warmth/cool ratio. Wear it when its chilly, but don't worry because it breathes so well that when you warm up it won't feel overly hot. If you only had to choose one piece of layer clothing to get, this would be it. There are a couple of other weight availabe, 150 & 250, that I'm sure would also do the trick. I'm a 5'6" guy but extremely buff, and the small fits great for me.
I wear this all year round, but it works greatest when its a bit chilly and you need that extra layer.