I've signed up on it. I've heard of people getting professionally connected through it. If you work in an industry in which people move around but you cross paths with often, or an industry where it seems that "everyone knows everyone", then it can be very useful. Railways being one of those industries, for example.
I know people I used to work with that now work for Alstom, Bombardier, Tube Lines, Network Rail, Delta Rail, West Coast....etc.... It's amazing how you end up crossing paths after years. Linkedin seems to be a useful way of formalising some professional relationships and who knows where your career will end up. It's not as trivial or casual as facebook and it's garbage in - garbage out. If you put a lot of effort into it, it might just be at least the initial stages of future employment. You'll still need an old fashioned CV, covering letter, good background an ability to interview though - those things will never change.
