I know couple of people who had a headache watching Avatar and several people to whom 3D made no impression. I couldn't see much difference between 3D and 2D. I barely noticed 3D in the film and it didn't make much difference to me. Those few moments when I noticed 3D it felt very unnatural, more like a CGI movie. Wasn't worth the extra money IMO. But probably I just couldn't see 3D as it is meant to be (it's not uncommon)... I did like Avatar though, only 3D didn't make impression to me.
Also, according to some studies, there could be more people who can't see 3D properly than those who can. Sure, most can see some kind of 3D but not even nearly everyone sees 3D as it is meant to be. So, that is also going to hurt 3D TV sales a lot.
If you don't believe me then read this: "56 percent of people aged 18 to 38 have problems that could make it difficult to view 3-D properly. Another 5 percent of the population have problems that make it impossible to view in 3-D."
Source (I wanted to find original research but couldn't find it): http://hothardware.com/News/See-Avatar-Diagnose-Your-Vision-Problems/
