My wife had a professionally applied water repellent treatment done on her new windshield.  She raved about how the rain droplets just fell away when she was driving.  Didnt need to use her wipers while moving.  I saw it work and I was impressed.  Based on customer reviews I bought Rain-X.  Following the directions I wiped on two coats.  I live on the Northwest Coast so the rain is without mercy here.  My first drive with Rain-X on the front and rear windows was incredible.  This stuff works and its just as good if not better than my wifes shop applied coating.  When moving at 25 mph (or 40 kph) the water simply slides away.  No need for windshield wipers.  I have a large rear window on my Impala with no rear wiper.  Dont need it.  The rear window stayed clear.  This is a first for me when driving my car in the rain.  Ive read the negative reviews for Rain-X. Its obvious that those individuals didn't apply the liquid coating properly. Its unfortunate that some people will critize a product that they didn't use correctly. Anyways, Im sold.