Adds breathability when hot, a little bit of insulation when riding in colder weather. Super soft material, and no underarm chafe — very important
Good wicking. provides extra insulation for early morning chill, but not too hot when temps warm. Sizing recommendation was spot on.
So far I've used this between 55 and 85 degrees Fahrenheit (12-30*C), it's a little warm at the 85-degree end of things for me, but I tend to run really hot. Wicks sweat well, nice tight fit, so no bagginess or flapping under your jersey, comfortable enough to wear all day. Went with a Medium, I'm 6'1"/165lbs (185cm/75kg), around a 39" chest, fits spot-on for me, just tight enough.
Has been super versatile and durable. Has held up great over the past year and is great to wear both in warm and cold weather. I sized down from my jersey for a snug fit.