It depends on what you mean by scared.
If you're ever felt panic due to heights, stay off.
If you're just nervous and uneasy but it doesn't ever get beyond that point, then I think you're ok.
How are you on airplanes when you take off? If that's bad, stay off.
It is impossible to distinguish how far off the floor you are once you get up. The place is dark. But you do know you're pretty far off the floor, and even though you do have a belt, your feet do dangle.
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It also moves you into position quite quickly and the apparatus banks right immediately as the movie begins. The shots on the scene look and the apparatus makes it feel very realistic. The opening scene has you flying over the Golden Gate Bridge from a fairly high vantage point and obviously the musical score starts quickly too. So it does feel like you're a good ways up. The simulator movement is very smooth though and never did it feel like I didn't have something solid underneath me.
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So really it depends on how scared you are. If it's a recurring problem or it gets to the point of panic, stay off. I wouldn't want you to panic, rip your own belt off, and jump off the thing in a panic induced attempt to get out of there. If you've never had those kind of problems but you're just apprehensive, (and note I am apprehensive of heights and stuff like that) and the opening scene really did spook me a bit. But I just took a deep breath and enjoyed it. Also, try to look straight at the screen rather than exclusively down at the bottom of the screen. You get far more sensation of movement if you look down near the bottom.