Its a bit too big for my 21 12 inch head but I like that it still fits above not over my ears so it's more to my liking for a cap under my helmet. It absorbs sweat so it works well in hot or cooler weather.
Living in Hawaii, I do most of my riding in hot and humid weather. The ultraLight skull cap helps keep the sun off my bald head and delays sweat from dripping in eyes. It's a bit expensive for what it does, but it is comfortable. Just don't forget to wear your helmet with it. I've left the house with just the feel of the cap on my head, thinking it was the helmet.
Does what it's designed to do well, wick away sweat, provide protection from the sun. I like the Pearl Izumi (discontinued) skullcap a bit better...higher SPF, nicer fabric, etc.
This cap is great when I'm doing a longer ride on a hot, sunny day and don't care to get a sunburn through the vents of my helmet. It's light enough to not be too hot and does the job of preventing my scalp from getting burned.
Fits easily under the helmet. Hardly know I am wearing it but I feel reassured I am protecting my bald pate. Love that it saves me from applying sun block to my head and waiting for the eventual sting as perspiration and gets into my eyes.