Here in the humid DC area, they are not cooler than bare arms in direct sunlight when you're not moving, but when you add the breeze you feel while riding, especially downhill, or you go into the shade, you can feel that they are cooler. I have also found that they keep my skin dry, even when the rest of me is drenched with sweat.

My only complaint is that they tend to yellow over time, and I think this is caused by contact with sunscreen. But every white cycling top I've worn has the same problem.