I'm not a fan of the looks of the window visors but I can see where they would be handy in places where you get sudden storms and want to leave your windows cracked on a warm day.
My 2002 Altima had one at the front of the sunroof when I bought it new, and it was the first thing that came off when I got it home.
When I lived in So Cal I would pop up the sun roof, if the car had one, or just crack the windows so the interior wouldn't get to 150* when it was well into the triple digits, temperature wise.