I've had my arms burnt badly on several IM triathlons since the sunblock eventually sweated off, either on the bike or run. These solved the problem. They weren't buky or bothersome and transitioned to my jersey. I didn't notice them, and I didn't get burnt (in South America during their summer).
Of course they were-they're Castelli....still, they really weren't as comfortable as I hoped and I tried multiple sizes. I ended up going with Pearl Izumi-cheaper, more comfortable and less visible logos. I love Castelli and own lots of gear, but these didn't do it for me.
Though more expensive than competitors, the material is a better quality. It is not hot even in direct sunlight and over 90 degrees. Much better than a thick layer of suntan lotion that may not work and allow direct sun making it feel hotter.
having just survived stage 1 melanoma surgery these are now my go to product for sun protection. no more slogging on icky sunscreen multiple times during a fun ride. yes they are very lightweight. they wick moisture away very adequately. got the white ones for extra coolness but they do show the dirt more than the black ones. i use these sleeves for both long distance riding and hours of tennis. i am 5'712 128 lbs and the medium fits very nice. i was concerned about the length for my long arms and these work perfectly. they are a bit pricey but worth every penny to prevent a return of any cancer.
These skins will be used to shield the sun on mild days when heavier warmers are not needed. Of course Castelli quality and fit are a bonus. They fit in a jersey pocket without bulk.