I used Rain-X for many years and have no complaints about it. I mostly don't now because I simply don't have time to clean my windows often, much less do a Rain-X install.
What I have noticed, is when washing my windows, using paper towels I can never seem to get them "film-less". The only thing I've used that works is a squeeky clean cotton shirt or sock. Almost every paper product I seem to use lets a film, but the cotton cloth buffs it right off. Not sure if it's the window cleaner (Windex, et al) or the paper chemicals. Could be the reason some people have trouble with Rain-X.