This skull cap is supposed to keep sweat from being a problem. I live in Houston and ride year round. It's hot and humid here in the summer - all 7 months of it. I use this skull cap every time I head out on a ride and I've never been bothered with sweat running down into my eyes or any other part of my head. I'd say this product works great.
I bought this to snug up the fit of my helmets. (My size is kinda in between L and XL) Interestingly, the Coolskin Cap has been warm on cool mornings and cool when it's hot. It's not enough to warm you alot, you'd need something thicker if you need real warmth. But for all the in between temps, it's been great. When it was 80, the cap got cool with the helmet vents open, which was what I wanted. The seams are in good places so it doesn't create pressure points. Nice amount of elastic in the edges so it's not too tight but stays in place when putting on and off my helmet.
The other reviews on this are accurate but I bought this strictly to hold my oxygen hose in place while putting on my helmet. I use the standard nasal oxygen hose that everyone wears hooked around their ears and the skull cap protects the oxygen hose from being pulled off of my ears when putting on my helmet. FIVE STARS ALL AROUND!!!
I wear a size LARGE helmet and this skull fat is REALLY tight on my head. Keep that in mind before you purchase. I also was wanting something to cover my ears, and this product doesn't really do that. Maybe it will stretch with time and wear though.