I play in 2D to keep my battery up as long as possible (but I then keep it on brightness 5 w/o power saver on so heh talk about conserving battery life, right????).
I play in 2D. In order for the 3D to actually look nice, I have to hold it away from my face. But if I do that, then I can't see the game because of my terrible eyesight. I don't really mind, though.
I play in 2D, I rarely play in 3D as it hurt's my eyes (I wear glasses). However, I've been thinking of getting the N3DS as the 3D is much better on your eyes. I should have gotten a 2DS as I rarely play in 3D though...
Always 2D. Always. The main reason it took me so long to get a 3DS was because the gimmick bothered me and I didn't realize you could turn it off - I get major headaches if I look at 3D stuff for a while so I was thrilled to hear I had the option of choosing to play without it on.