I have 3 of these shirts now (albeit older versions - as far as I can tell, not much has changed), and they're my go to shirt for traveling, casual summer hiking, occasional winter hiking, and to wear on hot summer days.
I brought two of them with me to Japan last summer and the shirts easily dried overnight on a clothesline despite the near 100% humidity. The shirts are quite for outdoor activities, but they have a casual enough appearance that I don't mind wearing them to school, especially on hot days.
While the Mountain Tech is lightweight compared to a regular cotton t-shirt, it is heavier than some other tech style shirts (the Stoic Breathe 90 is significantly lighter), and as such, I tend to use other shirts for my high high endurance activities (trail running, hard hikes, etc), although, the Mountain Tech will certainly get the job done. I've wound up wearing this shirt on a lot of rock climbs and the fact that it's a bit thicker I think has allowed it to hold up well to abrasion.
I'm 5'7", 155 lbs and the medium fits me well, but is just slightly loose. Which makes it perfect to wear casually, but still fine to wear on the trail.