No one outside the Valley does (figuratively speaking).
It's a thin social layer on what's just a Q&A site not that different to Yahoo Answers, which I guess is fine but I just don't see it going mainstream.
As an aside, I've always said--and I maintain--that I don't see Stackoverflow/StackExchange going mainstream either.
Ultimately I see the end for both companies being a Google or Facebook buyout in the $X00,000,000 range, which I think says more about the overall market than it does their inherent value.
EDIT: let me add regarding SO/SE that I think the SO model works great for programmers but my point--which I didn't put very well--was that I don't see that same tagging/voting/self-organizing model necessarily translating that well to other verticals. I guess time will tell.