The figure I've heard is about 50% of people experiencing some effects, with about 10% showing severe symptoms. There was an article in
Science Daily a few years ago about some research done on the subject.
http://www.sciencedaily.com/releases/2008/05/080521112119.htmThat may well be a conservative figure; a later article in
Air & Space Magazine put the number at closer to 70-90% of astronauts feeling some symptoms.
There some interesting and amusing stories connected with this phenomenon. Dr. Millard Reschke, chief of neuroscience at Johnson Space Center, has expressed the opinion that the three astronauts of Apollo 7 were all profoundly space sick, but in those macho-test-pilot days, they claimed the problem was "head colds". Nausea would certainly fit with their refusal to wear helmets during reentry; nobody wants to barf with a fishbowl over their head.
The weekend before Apollo 11 went up, Michael Collins went up in a T-38 and spent time doing violent aerobatics in what he called a "poor but hopefully adequate imitation" of the inner-ear abuse caused by zero-G, deliberately pushing himself to the edge of nausea before backing off. He knew from previous Apollo astronauts that any symptoms that appeared tended to be worst during the early part of the flight, and he was very concerned about the possibility of being ill during the time when they would perform the transposition and docking maneuver (to dock the LEM to the nose of the CSM), since as CM Pilot he was the only one trained on the procedure.
And finally, when Senator Jake Garn went along on a shuttle mission in 1985, he was
sooo sick that he was memorialized by the "Garn Scale", an unofficial scale used to measure one's degree of misery. Garn was no stranger to flight (he was an Air National Guard pilot with better than 10,000 hours in jets) but he still spent pretty much the entire mission "velcroed to the wall" as some of the other crew members put it.