My personal opinion is they work on race cars, but aren't beneficial on a street car as you have to be going way over the speed limit to make them effective on the street, or have the wing so aggressive that it would cause a lot of drag.
Very cool video showing how functional they are for cars on the track.
personally (and not knowing nearly as much as others on the subject), I was totally in awe of the difference it made. He looked like he was driving on butter.....