Have only worn this inside but was uncomfortable enough to not recommend. The shirt itself feels very warm - just putting it on felt immediately like an oven - but the seams, oh the seams. Sure they make the shirt look "cool" and "Stoic-y" but because they're, how do I put it, not flush with the shirt, the seams make the shirt absolutely uncomfortable. The feeling is as if something foreign is inside the sleeves constantly rubbing distractingly against me.
And for sizing, I'm 6'2", usually a medium (skinny) and got a medium but the sleeves are long even for me (and I have long arms).