Ale,
I've used Rain-X successfully too in the past, but mainly on crash helmet visors where it definitely helps stream rain droplets off as described by others above.
For me though, getting wiper alignment right also has the added benefit of improving the ambience in the car, especially on those days when conditions dictate you've got to have the hood up... no nasty juddering from the wipers.
P.S. There may be those who of course never, ever put the hood up... but they probably don't need wipers either, wear their underpants outside their trousers, and go under the name of Mr C Kent
