They do work. However they won't work for all cars. They work best on cars that have a blunt rear end that is almost as big as the frontal area. Before they were commercialized the inventor tested them on a Honda Civic in a wind tunnel and was able to prove a 4 to 6 percent drag reduction.
http://www.airtab.com/MicrosoftWordHondaWTReportV6.pdf
Looking at Neil Blanchards photos I don't think he used enough of them and might have had them too far forward. The install guide says they should be positioned 4 inches from center to center(Note that this distance can be increase some to accommodate mounting obstacles and lessening this spacing causes the vortices to interfere with each other ). It also states that the front of the air tab should be mounted 14 inches or less from the trailing edge of the vehicle.