Being Stereoblind, I never got the appeal. The only obvious thing happening when turning on 3D was the instant dip in image resolution.
For the longest time this effect was super intense for me for the 3DS power light, and it made it look like the light from the power LED was just piercing through the case of the 3DS, while the cutout for the LED was almost pitch black.
(for people with glasses and iPhones: look at your homescreen, look at a notification dot. Put the phone at the edge of your field of view. Notice the number dance around in the space. Works with Discord notification dots on a desktop screen as well).
I have a strong prescription (-7 in each eye, and astigmatism) and the 3D effect worked flawlessly, at least on the revised "new" 3ds model. As far as I know -7 is considered a "strong" prescription.
Though some games did cut the framerate in 3D mode
Thus why the 2DS undercut so many sales for the 3DS.
The Switch is probably the only real exception, currently; but I could just be ignorant and there are much better mods for it.
Back in the day we played the hell out of them because we didn't have anything better, but the original screens were truly awful even for their era.
One thing I remember is that it made it much easier to interpret scenes with effects like particles, glowy stuff, smoke etc.
At least by Dec. 2018 the 2DS/New 2DS made up about 20% of 3DS family sales.
My problem with my New 3DS XL is that the power button is right where I put my pinky finger when using the shoulder buttons so playing Super Metroid sometimes ends with me "setting the 3DS down" on a desk while playing it, and my pinky bumps the power button which immediately autocloses whatever app you were running. Maybe there's a config option to not do that lol