the 3d aspect of these are proving to have negative effects on the eye over perlonged use. Its all to do with how the eye and brain together percieve depth.
Extreme tests show that the eye and brain adapts and changes. In an experiement glasses where used to flip the eyes image ( which is already upside normally the brain puts it right) so for a while you see upside down. Over a short period of time the brain corrects this and you see the right way up. Remove the glasses and with nothing you actually see upside down again
The 3D can fool the brain into requiring an adjustment
Tests have also shown that in children under 6 it can damage eye developement
http://www.mobilemag.com/2010/12/30/...hildrens-eyes/
http://www.technologyreview.com/computing/24976/