Coolers, not Warmers
I actually find these to be a really useful item...but as Arm Coolers, not warmers. They're super thin, offer UV protection, and do a decent job of wicking moisture. I wouldn't say they're cooler than no sleeves on a hot day, but if you're worried about the sun, these will help keep you covered without overheating. On really hot days, try pouring some water on them. I definitely wouldn't classify them as arm warmers, though.