Plexus & Honda's Spray Cleaner & Polish will do the same thing as Rain X on the face shield... It may be a little depandant on helmet designs, but with the ones I've used, you have to turn or tilt you head a little to get the wind to blow the droplets away.
The nice thing about the Honda stuff is you can wash your bike with the stuff, too. My bike only gets a water bath about once a year, the rest of the time, it's Honda's spray stuff! I keep a big can in the garage and a little can in my tail bag (along with a little can of chain lube).