MS is unlike any other ride anywhere. It's now my favorite at WDW, I've never experienced any discomfort on it- but I have a cast iron stomach. On our last trip, I went on it right after eating lunch (that was the way my FASTPASS worked out) and still had no ill effects. But I know many others have; it's the only ride I've ever seen that has airsick bags right on board! Other members of my family rode once and enjoyed it, but will not ride again; it left them queasy for hours. Not me; I used their FASTPASSes once and rode 4 times in a row.
What interesting is that while you're on the ride you have NO IDEA that you are "spinning". You feel forces of it, that's what great about the ride. But do you go "upside down"? I don't think so, but I really don't know. It's the ultimate simulator, you really feel the effects of what you are seeing on the screen in front of you. But there is nothing jarring about it, like on Body Wars or Star Tours. Just the G forces you'd experience on actual space flight.
I think everyone should try it, unless they are absolutely sure it will upset their stomach. The only close feeling would be on those centrifuge type rides at amusement parks where you stick to the wall. Otherwise, follow the instructions, keep your eyes open, head back and face forward, and have the ride of your life!