The first generation was terrible. The 3D in the "new" 3DS line was vastly, vastly improved through the addition of eye tracking. I went from keeping it permanently off to permanently on with the swap between generations.
It worked well for me. It was like looking through a window into another world. I like it much better than the helmets that block out your senses.
They should really try this concept again with a more powerful system and higher resolution screens.
Agreed, and more research and user testing for everybody. It seems like some types straight-up can't appreciate it due to their physical characteristics.
I think most people left it off but I loved it, even on the original model (though the New models were of course infinitely better). It was like playing a little diorama (especially for games that leaned into that like the port of Captain Toad)
Similar for me - anything but the lowest setting was headache-inducing, and the lowest setting usually just made it fuzzy. But only in games, the test screens in the settings worked fine.