Never.

Depends on the game. Some look better than others in 3D, and in some it helps your depth perception. Super Mario 3D Land, always. Paper Mario, always. Both those games just look so vibrant in 3D, and it makes 3D Land way easier. Animal Crossing, only if I'm plugged in. Resident Evil, never. It looks terrible, and when the 3D is off it kicks in the anti-aliasing, which makes the game look gorgeous. Luigi's Mansion, also always. It just adds so much depth, turning these crummy little low-res images into things that look full and real--like looking through a window rather than at a screen.

It's weird too, because I hate 3D movies. But I definitely like it on the 3DS.

they did add a little notch in the XL model so now it has the same slide up and down but if you want it off the slider clicks into an off position, far harder to accidentally slide up.