Fwiw, I've used Rain-X for several years on the front windscreen - and used wipers & screenwash - with no real problems. At low speeds (below 40 mph) the rain "beads" and, although it stays on the screen, it causes less obscured vision than without it. At higher speeds the wind drives the rain off quite well. Wipers are still needed for best visibility, but the benefit is worth it.
Yes, road crud (diesel, oil, salt, etc) will cause it to 'break down' and the more you drive, the more often you'll need to re-apply it (up to once a month in winter), but even so, it's far better than not using it.
Spider
