I was able to play with an Nintendo 3DS, and I know exactly what these reviewers are complaining about. If you look at the screen straight on, the 3D effect appears almost effortlessly. But if you tilt the screen a little, suddenly you see double ghosty images on the screen, and it's disorienting, because your eyes don't know what to focus on. It can cause eye strain.
For video and images, I don't see this as a problem. But for playing games, where it might be natural to tilt the phone to control the game, that could be a problem.
Valid point. When I would try to view 3D content off of a paper I would shift my head but not the paper to view it at an angle. I can see why you are saying its hard for video game players to play a game in 3D. I just hope as you play the game more and more your eyes can keep up with the shifting of the phone. Probably not possible and I'm no eye doc.