The best short-sleeve baselayer
Capilene is the best fabric for baselayers other than perhaps high-quality merino wool. I prefer thin base layers because I can more easily later them (long-sleeve over short-sleeve) and they dry quicker.
The design on the Cap 1 S/S is great, too. Like its long-sleeve cousin, the torso length is also slightly long so the shirt can be tucked in to prevent drafts. The torso is also tapered for an athletic fit, and the sleeves are long enough to be there, but not long enough to needlessly flap around.
I use the Cap 1 running in warm and hot weather, and in spring/fall under a Cap 1 short-sleeve.