It's decent. For me seems to last about 2 weeks tops.
You want to clean your windows well before applying so the RainX can adhere to the glass, not a layer of dirt.
RainX does leave a greasy looking haze behind that can be buffed out but it takes some elbow grease.
There's another product called
Aquapel Applicator. It's quite a bit more expensive for the # of applications you get, but supposedly it lasts much longer. I've never used it but the reviews say it's great.
I just bought a bottle of Invisible Glass with Rain Repellant. (Sounds smilar to RainX). I'll let you know what I think when I get a chance to try it.
Some people have said they use sealants like Klasse High Gloss Sealant on their windows. I may try that when my new car comes in. (Anyone with experience, let me know if that's a good or bad idea).