I think they are very useful. HOWEVER - let's not make them an excuse for cars designed without visibility. Nothing beats as window you can see out of in your peripheral vision and rear view mirror. I hate driving modern cars with the visibility of a tank. I notice this when getting into the 993 - it is wonderful how much glass there is - I can see everywhere ...
The old bimmers are exspecially pretty in this regard ... I guess it isn't crash safe and has poor aerodynamics?
G
