“We have shown Nintendo 3DS at multiple events around the world to literally hundreds of thousands of people and have received very few negative comments. As with anything that requires focus, from reading to staring at a computer screen, taking breaks frequent breaks is always recommended. When viewing any kind of 3D images (TV, movies etc.), some people might experience minor discomfort. The effects are short term and have no lasting effect, in fact most players can continue after taking a short break. The Nintendo 3D was designed with a 3D depth slider that lets users adjust the 3D effect to a level that suits them for optimum enjoyment.” - Nintendo statement
I haven’t received a single headache or sick feeling from playing the 3DS, and I usually have the 3DS slider at max. We’ll see what the rest of our podcast crew has to say after they spend a week with the 3DS.