Like so many, I landed here via a Google search for "how to remove rain x". My girlfriend was driving home on a rainy night recently, and couldn't see because of the (improperly applied?) Rain-X on her '06 Mazda Tribute. I had the same experience years ago with Rain-X, and I had to endure it for months and through a couple of sets of wiper blades. I've never considered using it since, but my GF was at the local oil-change shop and they applied it last time she was there.
I'm an engineer and I'm also really fond of my lady friend, so naturally I wanted to solve this problem. Years ago, I acquired a special chemical for another project I was working on. This stuff seemed to be the right stuff, since it calls itself "silicone remover." (watch out, Pamela Anderson!

Sorry. I couldn't resist.) Well, it worked! It's about the consistency of light oil, and it smells slightly of oranges. It's called "Amtex CCR," and I got it from Amtex Chemical in West Chester, PA.
I wet a paper towel with the stuff, being careful not to overdo it and drip it anywhere. Then I rubbed it on the windshield, getting it nice and wet. I followed up by wiping it off with a fresh paper towel until the glass was dry. Then I sprayed glass cleaner on the area to clean any residue. I could tell right away that it got it due to the way the spray did and didn't adhere to parts of the glass. Problem solved!
I like this for many reasons. First, SHE likes it. Second, it doesn't involve dousing the car in lighter fluid, gasoline, acetone, or kryptonite, and it doesn't involve getting a belt sander, scotchbrite, steel wool, or other abrasives and attacking the glass with it.
Now I'm nearly out of the stuff, so I'm going to check on getting more. Never mind its other uses, but use your imagination.
-M