I'll give you the skinny on Rain-X.
Basically, it forms a semi-permanent bond to the windshield and is basically wax for your windshield. The bong to the glass makes the glass super slippery so that when you're driving, the water just beeds up and runs off the glass. It really only works at speeds of 40mph and higher so it's great on the highway. But don't think it will make your glass totally clear....you can drive on the highway without your wipers but the glass is not going to look dry or anything crazy like that. Around town and at slower speeds it does nothing, so it definitely will not take away the need for wipers.
The stuff lasts for a few weeks up to 3 months, depending on how much driving you do in the rain. So once the coat gets "weak" you just re-apply the Rain-X which is pretty easy and the stuff polishes up the glass too. One downside of Rain-X is that it makes your windshield fog up pretty easily.
Rain-X is basically one of those products you don't really need but once you start using it, you just continue to purchase and use it again and again. If you just keep your glass clean and replace your wipers regularly (and clean your wipers when they get dirty), it's cheaper and easier.
I would recommend Rain-X for only the following conditions:
-You're too cheap to replace your wipers
-Your wipers are broken and/or are not working properly and you need a temporary "fix."
Basically, Rain-X is worthless if your wipers are working fine and are in good condition, but it is a great product that should not be overlooked. Aside from the uses I stated above, it's great for other things. My father uses the stuff on his fishing boat because he often gets rained on while out fishing and his boat doesn't have any windshield wipers. I like to use the stuff on my headlights so that most of the water runs off for maximum illumination onto the road. It would also work well for the rear windshield on most cars (since most cars do not have a wiper there) so that the driver has maximum visibility of what is behind him/her.
If you find a good use for Rain-X it is definitely a great product.