I have a love/hate relationship with Rain-X.
I apply the original stuff to the windsheild every so often, but it is required to be applied at or above 4 degrees celcius, when it is dry. HArd to come by days like that in the winter up here.
So, in between, I run the Rain-X windsheild washer fluid from Canadian Tire up here. This is where I have had the problem. In the Taurus, I have never had a problem, aside from the 'misting' that follows a blade stroke. In my GTI, however, in the cold of winter, I have had the fluid separate, leaving a slimmy residue all over the windsheild, to the point where I couldn't drive the car without flushing the entire washer fluid system.
EDIT, I should also add that the yellow Rain-X fro CDN-T isn't a terribly good de-icer either...
A respected body shop up here tells me that Aquapell is everything Rain-X is and more, but costs $200 for a one-time application....