My brother started with Coraline, then went to see Avatar because we both read articles about how much better and viewer-friendly it was. The results were the same; my brother and I had to leave during the movie when his headache became severe. The manager refunded our ticket prices and issued tickets for any 2D showing. He said about one in three showings someone is unable to handle the 3D.3-D movies give my brother severe headaches. We, therefore, do not go to 3-D showings and do not purchase them. I wear glasses and, although it is claimed they fit over glasses comfortably, they do not.
I had headache and dizziness problems with the old-style 3D technology (which most live action movies use now, and which virtually all 3D movies did before last year), I suspect because of the artificial viewing plane they attempt to construct, but the Avatar technology - which is based on how the eyes and brain produce natural stereoscopy - didn't cause me any problems. Unfortunately, only Avatar and several cartoons (Toy Story, etc.) have come out so far that use it. I think Resident Evil is supposed to use it, but I don't have any interest in the movie beyond the cinematography, color palate, and Milla Jovovich (who is always good in action roles).
While that may be a small percentage, there are those who cannot tolerate 3D. My complaint is that the glasses are not comfortable over my glasses, therefore, my eyes do not become adjusted to the view (because I am constantly fiddling with the glasses to try to make them comfortable). Home viewing of Coraline was not any better, he still got a headache.