Never!As a person with emmetropia the 3D effect they way it is on old 3ds models,causes me dizzyness but now with my new 3ds xl I'm using it and I will always do!
It's so perfect and stable,the face tracker does its job,I'm in love with the revamped 3D effect on New 3ds xl!The 3D effect is also way better than it is on the old models because the new models have a dramatically improved screen in all aspects!

I usually keep it off, mainly because I forget to turn it back on. For games like Smash, it gets in the way. But for games like Zelda or Mario or Animal Crossing, I like to have it on if I remember.

Always off, graphics are half res otherwise.

What is this, a Club Nintendo survey?

I voted "rarely" because I have the 3D turned on less than half the time I'm playing, but I don't think that's an accurate word to describe my usage. Maybe "intermittently."

I never use it... orz I heard it drains battery life, so I keep it off, and keeping the screen on focus is too much hassle for me. But it does look really cool when I use it : D

Before the new 3ds i rarely used it.Now I always use it.

Lol, that is what I thought of at first when I saw this question :P

I basically always have it on, the only exception would be with pokemon because the frame rate stuttering really bugs me.

On the old 3DS I never used it, but on the New 3DS I do turn it on sometimes!

Not really, it gives me headaches.