Just took the Boreas on a 9 day trip in Sequoia-Kings Canyon area, and liked it a lot. Wore it every day, slept in it when I didn't have time to bathe (wore rain gear over it to increase warmth and protect sleeping bag from funkification). Had a wide temp range, I liked the high SPF, the snug fit was great under a backpack but never limiting, it cut the wind well, and seemed to wick moisture away well and cool me when exertion/temp got high.
Normally would have used a thin, fine weave, non-lycra shirt, Adventure Racing or the BPL shirt, both lighter, but in this environment--high altitude, potential for cold & wind--the Boreas meant I rarely had to layer anything over it. Seemed to handle sweat better than the fine-weaves, which can feel a bit sticky. In any kind of breeze, the Boreas dried very quickly, and was cooling.
I wouldn't choose it, though, where temp/exertion was always going to be high, and the russet color left a bit of stain on my cuben fiber backpack, no big deal.
In fall or spring, this will be what I wear in the High Sierras.