first time I rode M:S, I was so dizzy I could barely stand. My wife rode it with my son (parent swap) and by the time she came out I was about 80% again. A little while later, all better. My son and I rode it 9 times on that trip, and I only got dizzy one more time. (it was my fault, they say not to look around, but I had to be a wiseguy...)
I've probably been on it 20 times since it opened, and it's something I ride to make my son happy (wife won't go on it again) but I don't really enjoy it that much.
The scariest "ride" at WDW is by far SUMMIT PLUMMET in Blizard Beach. When you come over the edge, you lose contact with the slide for what feels like 10 seconds and freefall until the slide comes back under you. Knowing that your journey ends with a 55MPH wedgie doesn't really help either...
Steve