They work real well, my experience with them came about when my FIL bought a set for his wagon, they do what there designed to do, but IMO pretty expensive.
I had a go at making my own out of the reflective blinds which you place on the inside of your windscreen, I bought the stuff from supercrap, cut it all to size, and attached some small suction cups and worked prefect, prob not as efficient as the more expensive ones, but did the job..
Swanny