I just rode Soarin' last week and I was fine. I had SEVERE motion sickness that lasted 5+ hrs when I rode Mission Space, and I'm the type that gets sick in the backseat of big cars, on boats, etc (though I seem to do fine and/or recover quickly from Thunder Mountain, etc). At any rate, for the very first few seconds of Soarin (you sort of swing to the side over the Golden Gate bridge) I thought I might get sick . . . but the rest of the trip was fine.
Also, keep in mind that you can close your eyes if you start to get sick and you'll be fine. The movements of the "hangliders" are not jerky at all. On Mission Space, on the other hand, you are definitely MOVING and closing your eyes will not save you!
HTH