I have a few "Evap Cap" from Performance and I like them a lot. http://www.performancebike.com/bikes/Se ... m=evap+cap
My guess is that they're identical to the nashbar version linked above, except for different color combinations (I like full-white!) They are very minimal and not even noticable (to me) in hot weather. I do think they improve evaporation and cooling as opposed to feeling warmer. I wear them virtually every ride now. I do believe it not only functions for sweat evaporation and keeps it from running down my face and protects the top of the head from sun, but it also keeps the inside pads of my helmet much cleaner, fwiw. Easier to wash the skull cap than remove and wash the helmet pads.
I also have, I believe, a Headsweats coolmax cap. http://www.performancebike.com/bikes/Pr ... 1_302052__
It is just about the same kind of material, similar forehead sweat band, and it works fine too. What I don't like about it is that it has the flap of material dangling down the back. I don't know why those are so popular - the tied bandana pirate look? No funciton as far as I can tell. If the flap was a little wider, it could provide sun protection to the back of the neck, which would be good, but it isn't substantial enough for that.
Another reason I prefer the Performance offering is that it is more minimal, it just barely covers the top of the head with the sweat band in front about where the front of the helmet comes to on the forehead and is well above the ears on the sides.. The Headsweats cap is quite a bit larger. It actually covers the top of my ears. Might just be made for larger heads.