Simply put, the experience of playing a game in three dimensions takes effort. You’ve got to keep your eyes more or less parallel with the screen, tweaking the slider to match up the left and right images or else endure nauseating ‘double vision’. Even when the images are correctly aligned, processing the depth of view is a little taxing, especially in a fast, fireworks-laden rail shooter like Kid Icarus, with objects transiting rapidly from far to near.

I found that my eyes felt tired after as little as 15 minutes, though in fairness some of this should be attributed to my looking at a variety of titles in succession, each with its own graphical make-up and ‘sweet spot’. I imagine that timeframe would shrink were I standing in a dim bus shelter, or wedged in one corner of a tube car, rather than a well-lit, spacious chamber in one of London’s premium business locations.

http://videogamesdaily.com/features/201007/the-next-best-thing-about-3ds-turning-the-3d-off/

Click the link to read the entire article. It is mostly very positive, but worrisome that it could end making you feel nausous. Regardless though I am excited for it and will have it Day 1.

I think it is good that you are able to turn 3D off because I am one of those people that has significant issues with 3D.  Although I like the effects of 3D when they are done correctly, it's not really a huge selling point but still curious about 3DS (not really curious about the other 3D ventures though).  But I could imagine myself getting headaches from it so I'm glad there will be an ability to lower the intensity or turn it completely off.  But indeed, it might be one of the most important functions of it.  But any "good" thing should come in some form of moderation, and as gamers maybe we take that for granted too much.

I have no issues with 3D, and while I can't comment on the 3DS, this guy sounds very similar to the people who complain about not being able to watch a 3D movie for more than half an hour without getting nauseous.

Strange to say but the problem is because its glasses free. 3D work by having a distinct image to each eye. Glasses makes 100% sure each eye get the image signal at the right place, because each image is diffuse at a 180 degree angle while being block alternatively by the glasses fast enough that you dont notice it. You can move and change position and still get the same 3D without ghosting effect. While a glasses free device need adjustment, and if you move, which is going to happen while playing on a handheld device, the image will not focus at the right place.

Tough, this doesnt apply to all glasses free device, the TV made by phillips (technology too big and expensive to use in handhled for the moment) displays a distinct image to each eye inside a wide radius. It diffuse 2 images at multiple and different degree from each other. Those display doesnt need adjusment and will not give nausea or ghosting effect as it is a natural way of seeing 3D.

Philips tech:

I can imagine a lot of people ignoring advice and getting sick from the 3D, good thing there is a slider. 3D seems to make people more quesy the faster it is, so lets hope nobody throws up on their DS during a game like Wario Ware lol.

I bet loads of people wish the virtual boy had a slider

ok, let's assume the 3d wont work, that would be pretty sad. Because the main feature of the device wont do as promised and ultimatly the 3ds will be just an upgraded ds

There are a lot of people who get nausea even with glasses. I know several people who absolutely hates watching 3D because they get either headache or nausea and for some people it makes eyes hurt. I also know people who are unable to see 3D at all. Glasses don't prevent nausea.

Yes I know, I never said glasses 3D prevent nausea at 100%, its just less problematic. Particularly with LCD shutter glasses, the cheap polarise glasses used in cinema produce some ghosting, some will experiences headache and nausea mainly because of that. But most people doesnt get any problem using both glasses anyway... Theres always exception, my point was that for 3DS it may not be only a couple of exceptions. We will see when the times come. Sorry for not being enthusiastic, but by knowing how 3D work I have a hard time imagining it will work well in the long run. Hopefully, my fears are wrong, I would really enjoy a portable 3D experience. Ill buy it anyway and judge for myself when its release.

Portable 3D porn! ...wait.. this isn't a suggestion thread... *leaves thread*