Apparently Japanese people are complaining that it causes eye strain after 5 minutes of play with 3D turned on.
http://www.maxconsole.net/content.p...s-report-3DS-eye-strain-after-minutes-of-play
The Japanese company said on its website that children 6 years old and younger shouldn't play 3-D games on the coming Nintendo 3DS hand-held game machine, due for release in Japan in February. The company said looking at 3-D images for a long period of time could have an adverse effect on the eyesight development of young children.
http://online.wsj.com/article/SB10001424052970204304204576051021329863968.html