I've been using Rain-X for years. It's great stuff. As said in the first post, it makes your wipers work really well, they glide so much better.
I've recently tried the gel type but it's nowhere near as good and really difficult to wipe off thoroughly after applying it.
I've also tried several different makes but nothing seems to work quite as well as the genuine Rain-x brand.
Nah, that's not right. The screen needs to be cleanish just so that the Rain-x liquid can properly get to the glass and fill the tiny dimples in the glass surface. If the screen isn't clean when you're applying it the rain-x just doesn't work as well, that's all.
A coating of Rain-x only lasts about 3-4 weeks anyway, after that it gradually loses its water repelling ability.