We are just back and rode MS three times during our visit. My husband and I thought the ride was great-- surprisingly intense (couldn't believe Disney would subject guests to so many G's!). I have a fear of heights and would never, ever ride Tower of Terror or anything but a kiddie coaster. My husband rides all those types of rides, however, and said that the first ten seconds or so of MS was more intense than a coaster or drop ride.
Neither my husband nor I are at all prone to motion sickness, however, and I love all kinds of crazy spinning rides that make many people sick. I was a bit leery of Mission Space after reading posts on this site, but I am very glad that I tried this out. Mission Space is a really, really impressive ride and very different from anything I have ever experienced (at a theme park or elsewhere).
Use your best judgement, but if in doubt, you might go ahead and give it a try...
Beatnik