First, it works. But...
It's quite a bit larger and heavier than I'd imagined from the photos and description, so wearing it on the peak of a cap will be much less appealing than I'd hoped. This wouldn't be so bad if it really blew a storm, but even in direct sunlight the air flow it manages to create isn't that strong. It would be welcome on a really still day, but if there is already even a slight breeze, then it's not going to be very noticeable - so you have an almost pointless big lump on your hat.