I like it with Outlook 2013 -- there's a built-in plugin that basically looks up people that you communicate with on Linkedin. I find it handy as I meet with alot of people, and I get a picture and brief bio, and I get you see who they've "linked to" recently.

Before that, I found it to be completely useless -- sort of like a grown-up version of baseball cards... you collect people for unknown reasons.