Based on my experience:
StreetPass can be very fun and intriguing.
StreetPass can be downright boring like fuck.
I experienced both. When Japanese 3DS consoles were expensive like hell and Singaporeans didn't know much about the 3DS (due to zero advertising, 3DS = 3D DS), I rarely got any StreetPasses. The max I got was 2 at once, and that was in a city tech convention.
Now, Singaporeans are more exposed to the 3DS. Some who were unhappy with the Vita (I know some of my friends were) sold it off and imported 3DS LL from Japan. Many others depend on local stores. I get about 4 passes whenever in the city. The max I got was 13 at once - when I walked around Orchard (Singapore's mini Ginza).
Now I have about 40 tags, which was accumulated over 2 months. In contrast, my US console which I sold off during the 3DS low-tide season, had only 28 tags over 6 months.