Basically StreetPass lets your 3DS talk to other 3DS's as they get near each other. My Mii is sent to their 3DS, their Mii is sent to mine. From there you can play a couple of the preinstalled games (a puzzle piece swapping game and a mini-RPG) with the other Mii's that you've collected. You also keep track of where the Mii's come from to try to fill in a map and check off certain achievements (like meet Mii's from 2 different states, etc.).

Certain games (like Super Mario 3D Land) also incorporate the function with added bonuses (SM3L gives you extra toad houses, for instance).

It's nothing super deep, but a nice diversion and like others have said, it entices you to bring your 3DS with you when you leave the house.

I like it. I have 50+ right now, and it's fun to find out you passed someone that's rocking out the same games and interests. I opened mine once at a movie theater while in line, and I realized the guy next to me was added. We ended up talking, and he was a retro collector as well. It isn't super deep, but I think it's a fun addition.

I tagged 200 people in a day at a convention On regular travels I tag a handful of people on the weekend if I take public transit instead of driving or walk around a mall.