Graphics are half res either way... At least I hadn't a 3DS game yet which switches from 400x240 to 800x240 when turning off 3D. It stay at 400x240, stops rendering the second angle and halves the processing power (better battery life in 2D).

Most of the time I have 3D turned on, because I love the stereoscopic effect in most games. Exceptions: games with bad 3D (f.e. New Super Mario Bros 2 blurries the background), in situations where it's hard to keep the device still for a steady 3d-effect (f.e. traveling by bus) or to save battery life (if the battery is dying and I have no charging option near me).

Very close poll!

I've only had mine for about two weeks, but I almost always play with the 3D on. It's a neat visual trick, and I find it useful for depth perception. Besides, I'm not about to buy a "3DS" and not use the title feature! ;)

You do not understand the technology it seems.

The 3DS *ALWAYS* renders at 800x240, turning on 3D enables paralax barrier which splits viewing of even and odd columns to each eye, thus in 2D mode both eyes see 800x240, in 3D mode each eye sees 400x240, because there is no convergence the even and odd columns are overlayed on top of each other giving the illusion of depth, but the cost of doing this is cutting the horizontal resolution in half - this is why the original 3DS suffers so badly from the angle and distance of the viewer needing to be perfect for it to display correctly.

Most UI elements are rendered as normal in 2D mode, especially in the UI menu of the 3DS itself, to maintain that resolution

the 2DS battery life is better because it ships with a 3.6v 1300mAh 5Wh battery, where the 3DS ships with a 3.7v 1300mAh 5Wh battery, the 2DS doesn't need as much power because the upper screen is a standard panel and not a paralax barrier display, thus doesn't need the extra backlight brightness of the 3DS screen since it doesn't have as much display to illuminate.

It's very rarely. I used to a lot more when the system were new to me.

I used it a ton with Bravely Default and 3D Land, but hardly ever in other games. So, rarely.

It depends on the game. Most of the time, yes. But with Smash (a game I was taking super seriously at the time) I only turned it on during replays.

Never, I always have it off to save battery life and I was never personally fond of 3D.

I sometimes turn it on for about 20 seconds, which is usually enough to remind me why i had it turned off :p The feature itself is okay, but i don't feel it's worth the cuts, battery drain or infuriatingly small view window. I've heard the N3DS fixed the last one, but i have no interest in buying a 3rd 3DS.

Do you have any sources for that claim? I trust my own eyes. When I pull the 3D-slider slowly down, the angle between both 400x240 viewpoints gets smaller until they are virtually the same viewpoint, but both viewpoints are still rendered in 400x240 each. If the 3DS should switch from this minimal 3d-setting (2x400x240) to 1x800x240 by switching off 3D at all, there should be a clearly visible gain of image quality (and a gain to the resolution of the lower screen)... it ain't. The jaggies don't get smoother in one dimension, the 3DS stops rendering the second viewpoint in 400x240 and mirrors the 400x240 of the first viewpoint.

If 3DS games, apps or menus would support native 800x240 in 2D-mode, the 3DS should also have a better image quality on the upper screen than the 2DS: http://www.nintendo.com/3ds/features/compare

It hasn't. The 2DS has the slightly better 2D-image quality: http://www.golem.de/news/nintendo-2ds-im-test-mehr-2d-schaerfe-fuer-3ds-spiele-1310-102168.html 

"This new design has advantages in the 2D representation of 3DS titles. The uniform pixel structure and the apparent resolution of 400 x 240 pixels, the image on the 2DS appears decently sharper than on the 3DS in 2D mode. Since the 3DS alternatively offers 800 x 240 pixels for the 3D mode, it has disadvatages in 2D due to the design. It reaches its optimum sharpness only in 3D mode."

In this video 0:55 to 1:30 : https://www.youtube.com/watch?v=2uxX6S-IXLo

Who is talking about the 2DS? The 3DS and the 3DS XL both have considerably better battery life if they run games in 2D instead of 3D... because the SoC only has to render half of the polygons/pixels for the upper screen.

I use it often, usually for big events like the Pokemon league and such. However it can give me headaches so I turn it off when I feel one coming on.