Stopped buying any other kind of base layer when I found these. Offer the perfect amount of give and perfect fit. Can't recommend these enough.
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.