My side windows are constantly greasy. Even after a wash - if I put the window down and up again, it's all smeared on the inside. If that is this Rain-X product, I'd rather not have it.
Actually, I have a problem with my driver's side mirror being greasy, but it's not the water repellant coating. Some dealers lubricate the door and window seals with the good intention of preventing the seals from drying out and cracking. The problem is that it causes this greasy streaking on the side windows. Very annoying!
I tried Rain-X over the Porsche coating (at 4 months old the OE coating was all but useless). The RainX didn't seem to take very well, and was very dfficult to remove - always blooming on the glass when observed form low angles. I've put Rejex on now and its much much better