I used it on my other cars, but not the 4Runner. It's great the first week or two when it's applied, you literally don't need the wipers and it's fun just sitting in the car watching the water roll right off the glass.
After a while when the application wears out, water starts to form bigger beads and sheet a little bit, it hampers your vision especially if there is dirt (pollen or dust) on the glass. While the windshield is ok, because you have wipers, their effectiveness streaks a bit because the glass is semi slick. And it's terrible once the glass gets dirty, hosing the glass doesn't fix it (where on a non-Rain-X'ed window it come clean). I think Rain-X attracts dirt.
I say Rain-X is great as long as you keep up with the application and keep fresh coats of it on the glass. Otherwise, IMO, better to go without it.
As for applying it, I spray it on the glass and wipe down the whole glass. Then I take a dry towel and buff it until it's streak free. Usually, you can tell it's good because the glass is super slick and slippery.
Then there's this product,
Aquapel. I would expect it to be the same as Rain-X. I want to give that a try, but not if it's like Rain-X and becomes a PITA to remove completely.
I'm going to try applying a coat of wax on the side glass this winter instead and see how that goes rather than use Rain-X. I would use Rain-X if I can figure out a way to completely remove it when winter is over. There are times when it always rains or it's foggy and misty and reapplying Rain-X isn't possible.