I rode M:S with my FIL in May.
Everywhere leading up to the ride, and inside the antechamber, there are signs and announcements warning certain people away from the ride. There were four conditions, three of which were: prone to motion sickness, dislike enclosed dark spaces, and dislike spinning. Yes, yes, and YES. I still really wanted to ride, anyway. My Dad rode it last year, and Id also read it helps to focus on an object on the monitor and NOT close your eyes, and if I didnt already know it, Gary Sinese told it to me in the pre-flight video. All I can say is thank God for the cool air they blow across your face during the ride, because Im sure that was the biggest factor in my not needing one of the ample supply of barf bags. I loved the ride, as did FIL. However, I felt slightly dizzy for about the next hour, and even several hours later I felt a sort-of flashback when, of all things, I rode one of the elevators in The Living Seas.
FIL had a great time making fun of me all day, and since I deserved it I let him have his fun. He was clueless, however. When I told him how the spinning of the ride messed with my inner ears, he insisted we were never spinning and said the warnings about it must have been some sort of misdirection. I asked him how he thought they produced the g-forces we felt, especially during lift off. He said it must have been some kind of special effect. Since hes a pharmacist and I have a lot of chemistry classes under my belt, I tried explaining to him that in the ride we were in a giant centrifuge and that our backs were being pressed against the bottom of the test tube, but he didnt buy it.
Even though I ALMOST felt sick the whole time, I loved the ride and will do it again the next time I'm in the neighborhood.