For what it's worth . . .
I rode MS this past Saturday. DH (who has recently gotten very queasy on teacups and now opts out of anything that spins) sat out, so I went in solo on the (hooray, fast!) singles line.
Before entering, I asked a cast member if lots of people really do get sick. His answer, People get sick on very ride. Wasnt very reassuring!
About me: I'm a major chicken on most roller coasters, though admittedly it's not about feeling physically sick - it's fear. Its being dropped that I HATE (did ToT once, never again), though I enjoy spinning, turns, and almost all motion simulators. I never get car sick, can read in moving vehicles (though reading on a treadmill makes me very dizzy), and while I dont think of myself as having a cast iron stomach Im usually not particularly reactive. I do usually feel a bit disoriented and rubber-legged after exiting RnR. Rode in the morning (ride had been shut down for a while) and I didnt have a ton of food in my stomach, but had eaten breakfast about 1.5 hours before.
After reading stories here (and hearing--a bit of folklore perhaps--while at WDW that on a test run for cast members nearly everyone got sick), Ill admit that I was a bit nervous. Two women in front of me on the singles line, however, had just come off and were going again and assured me that they felt totally fine. . . and that the worst part was the warnings. They werent kidding! Ive never seen so many explicit announcements about backing out. Gary offered people the chance to bail out many times over.
I believe I was the engineer (far left in my module). Kept eyes open and straight ahead the whole time. It was a lot of fun, and I definitely felt the force pulling me back. But it really wasnt bad at all. Its a very smooth simulator andif Im really being honestalmost wished for more speed and more motion. Couldnt tell I was spinning at all.
When I exited I felt no ill effects whatsoever, but it was clear that others werent moving toward the exit at the same clip that I was. One woman was crying hysterically. I didnt personally witness anyone getting sick.
SO as someone who is pretty chicken about rides but who doesnt get motion sick I was 100% totally fine (only one ride . . . time/other commitments prevented a return visit). But I imagine if you know youre prone to sickness your body might know it was moving in circles, even if you couldnt see it.
Happy to answer any questions.
This is exactly what I was afraid of..I have been having nightmares about riding MS.See, I always seem to close my eyes by instinct when I'm on a ride.It started the first time I rode Space Mountain when I was 5 (long story).
. I think I'll enjoy the preshow, skip the ride and go to the exit to people watch while everyone else rides it.Maybe, after looking at everyone's faces when they exit ,I will not regret not going on. 
LOL 
