Used it on my old car. It's great the first week or two. After that, it dirt sticks and makes it worst. Normally, without Rain-X, dirt will just hose off, but Rain-X treated glass, it takes scrubbing.
I prefer to use Klasse All-In-One on the glass. Same effect as Rain-X, except it doesn't attract dirt or make dirt stick.
I find Rain-X to be like Armor All, once you applied, it you need to keep on applying it.