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.