I'm very prone to motion sickness. I also have an inner ear problem and have experienced vertigo (dizziness with a spinning sensation) a few times which left me with minor balance problems. So I cannot handle any ride that spins or jerks too much or goes upside down. However, thanks to some early reviews from folks on these boards, I tried Soarin' back in early May and I loved it!
First of all, Soarin' doesn't spin at all. You experience a sensation of soaring, not spinning, over California. It is wonderful! I was fine the whole time except for one 3-second segment featuring a very sharp banked turn over an aircraft carrier. I just closed my eyes for a few seconds and when I opened them again, I was fine. There is a wonderful breeze blowing in your face the whole time which really helps keep any problem with motion sickness at bay.
I was surprised to discover I do have a slight fear of heights, but kept that under control by just reminding myself I'm watching a movie, I'm not really soaring over a mountain peak and I'm tightly belted into my seat.
So, I feel confident in saying that if I can handle this ride, just about anybody with motion issues can handle it!