Never ever use this **** for a car windshield that is driven over the winter and where salt is used. When the combination of salt water/slush is thrown against the windshield and with the defroster heating the windshield, the RainX makes the water evaporate almost instantly leaving a white haze of salt that is difficult to see through; and which needs to be removed with lots and lots of windshield antifreeze. And if you get the antifreeze with RainX already added, then you really will have fun cause it's like trying to put out a gasoline fire by adding more gasoline!
For all other applications that don't involve salt water being constantly splashed onto it, like side windows, mirrors or visors, it's good to use.
If you want antifogging, then buy an antifogging solution, there are enough on the market, even stuff found in sporting goods stores that is used on swim goggles.