Very comfortable, surprisingly durable for how thin the fabric is, and very odor resistant. I use these as baselayers when traveling or in the backcountry (e.g. multi-day yurt trips) when I want to be able to wear things a number of times without getting too smelly. Warmer than a thin polyester or nylon baselayer, but still comfortable in a wide range of temps. The long sleeve and quarter zip versions are great, too.

I'm often between a Medium and Large depending on fit and brand (5'10", 180#, wide shoulders that often don't work with slim cuts). I've found that Smartwool's slim fit is pretty skin tight in Medium, which works as a tight baselayer but not as well for casual/travel wear. These are a little loose on me in a Large (a relaxed fit, perhaps), but not too big. Smartwool also makes a 150 t-shirt that's not a slim fit (the v neck, I think)—that one is too big in a Large for me.

I use both the long sleeve and the short sleeve depending on the needs. Just versatile.

I wore this shirt under my synthetic running singlet in this year's Boston Marathon, which experienced some of the worst running weather in history. It was cold, wet, and windy. While many people were dropping out due to hypothermic conditions, my core stayed nice and warm despite being soaked soaked through.

Warm, comfy, and and looks nice. A bit looser fitting the typically base layers. That's how I like it.