I live in the south with brutal humidity and I wear a Castelli mesh baselayer and I keep my jersey zipped down about halfway. It works as an evaporate air conditioning system that houses in the desert have. The air is cooled as it flows through the wet mesh. They absolutely work (or I wouldn't wear it). It keeps your skin from getting wet and clammy and sticky and your jersey doesn't get as soaked. They're light and thin, you can literally ball it up in your fist. It's one of those "don't knock it until you try it" deals.
I also ride sportbikes and in the summer I wear a cooling vest under my perforated leather jacket. It's a vest you soak in water before you put it on and it's like having a mini AC system.