I had a go on a 3DS a couple of weeks back as they had a temporary touring expo thingy in nottingham town center one weekend.
I have to say, I was VERY disappointed.
The 3D effect was awful, just kind of gave two planes -foreground and background - rather than any proper depth of field, I had to hold it at a funny angle to avoid getting a weird double-vision effect, and any slight movement would bring back the weird double-vision effect.
I had heard the visuals were supposed to be up there with the PS3 and Xbox 360. Whoever is saying that is full of s***. It's Gamecube / Wii quality. No better than the kind of visuals you can get on a modern smartphone, and not that much better than the ageing PSP.
On the front of the unit, next to the top 3D screen, is a '3D slider', which increases or decreases the 3D effect. I suspect after the novelty wears off (approx 5-10 minutes after playing on it) and the users eyes start to hurt too much, that slider will get turned off permanently.
I also anticipate a great deal of '3DS Ruined My Child's Eyesight!' tabloid headlines.