I agree with Douglas - I tried it many years ago and it gave the windscreen a greasy effect. Great on the side and rear windows but not easy to shift from the front if you apply it and don't like it.
Amazing stuff. At low speeds you still need to use the wipers, but from c.40mph upwards it works like magic. I understand that it was originally designed for helicopter windscreens where the downwash from the rotor forces the beaded water off the screen. Rain-X water repellent causes the water to bead on the screen and the wind blows it off.
It's brilliant stuff. I first used it on my motorbike helmet visor, so that must be 55ish years ago. If you do decide you want to remove it from w/screen use one of those washing up sponges with green scourer on back and hand washing soap, comes off easily. Do not use washing up liquid. Manufacturers of washup liquid use 40-50 per cent salt to thicken it and you don't want to put salt anywhere near your car.