Erick Rodriguezwrote a review of Castelli UPF 50+ Light Arm Sleeves on April 23, 2017
Familiarity: I've put it through the wringer
Castelli arm skins are a quality product. After two years of use they are still holding up as if they are new. The silicone grippers are in top shape and I love that it is a consistent band of silicone all around the arm opening. I own Gore Bike Wear windstopper sleeves and I've tried Pearl Izumi SPF protection sleeves. These two brands don't have a solid silicone band like the Castelli sleeves. It's a testament to the details that Castelli curated. Granted, the windstopper sleeves are intended for colder weather but the Pearl Izumi are for warm weather. I can tell you the black PI's felt warm when I tried them on whereas Castelli skins has a minty cooling effect in black and white colors. Regarding hot spots, there are none due to flatlock seams. I recommend cutting the washing instruction label, it bunches up and gets annoying.
For reference, I've used these in 105 degree weather last year and they felt great. Not as cool as when it's about 95 degrees but they dried quick and kept my arms from burning and tanning.
If you need any of these Backcountry is the right spot. My local bike shop has to special order them and if you lookout for specials, you can get a pretty sweet deal here.