IMO, it's pretty bad and I'm not prone to motion sickness. Granted, my stomach wasn't feeling too great to start off the day, but after riding FJ, I was done. This isn't a ride where just closing your eyes will stop the motion sickness. There is a lot of 'real' movement, in addition to the 'simulated' movement with the movie screens, if that makes any sense.
As a huge HP fan and one who enjoys thrill rides, I was actually a little disappointed in FJ. It was a great ride technologically speaking, but I thought it was a bit too much. It was hard to really focus on what you werre seeing, it all goes by so fast. We went with my 8 and 10 year old nephews . They rode Soarin, Test Track, Star Tours and Expedition Everest before we got to IOA and loved them, so we felt confident to take them on FJ. After FJ, the 8 year olds were afraid to try any new rides (one was even afraid to go on the Jungle Cruise) and even the 10 year old had no desire to ride the other 2 WWOHP attractions. Thank goodness we did IOA toward the end of our trip.
I guess I just feel that since the Harry Potter books are children's books that appeal to a broad range of ages that the ride should have the same appeal, a la Soarin and Star Tours. Granted, not every ride will appeal to everyone, but in IMHO, FJ missed the mark on it's prime audience.