I have used premium car glass sealant products for many years as a prestige detailer, and rate them highly. With the right sealant you hardly have to use your wipers at high speed, the water just runs off in beads. If you've not tried it I highly recommend it.
Ohh, and not RainX either, that's at the lower end of the market in terms of performance and durability.
Here's the best glass sealant on the market today, cheap and long lasting (6 months usually) There's a video down the page made by Tim, the owner of the website, you can see the difference in the treated and untreated windows side by side -
http://www.cleanyourcar.co.uk/engine-amp-exterior/carlack-glass-sealing-kit/prod_348.html