I don't know about RainX, but i use the turtle wax clear vue rain repellent. I have applied it twice since April this year and from 50kph (=31mph) raindrops are rolling from the windshields. I hardly use the windshield fluid, but clean the windows every other week with a spray bottle (demi) water or during a car wash (by hand).
Instead of using this on side windows, i applied the paint sealant there. Much cheaper. The Finish Kare 1000p sealant is a great winter sealant and can be used on paint, alloys and windows. Ice and dirt go off easy and its durability is 4-6 months.
Btw, i ordered that sealant special from the UK, because it's not sold here in the NL.
