I liked the StreetPass Plaza stuff. It was interesting enough for me to check out whenever I got some, but wasn't substantial enough for me to feel like I was missing out during the periods of not getting any passes. I also didn't buy any of the extra games, though I did get one for free that I used some.
As far as actual games go, I was never interested in it there. If those features could also work over SpotPass so that I could receive data from my friends list, I would have checked them out from people I actually know.