I have used it for years, it works great if your going over 40mph, the faster you go and the heavier the rain the better it works, the rain flies up the windscreen leaving it clear, one application lasts me for about 3 months and I have never had any smear on the windscreen, just the opposite.
I have driven from London to Cornwall in heavy rain and just used the wipers a couple of times to clean the windscreen.
To apply it you must follow the directions on the bottle, if you don't it won't work!
Would not be without it.