I'm a freelnace marketing consultant and I love it, and find it very useful...
I have 300+ connections now (ex school, uni, jobs etc and 'professional' friends)
I've joined various relevant 'groups' which help me keep up to speed with what's going on in my industry.
I've linked it to my Twitter account (work one, not personal) and I will post if I get a new client, am working on a particualar subject and want feedback, or if I publish an article, or speak at a conference.
It definitely has helped me get new business - this week someone called me who said they'd seen my article and like my POV, and could I do some research for them
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I keep LinkedIn very much for my 'professional' life though. it makes me cringe when I see people posting stuff about going to parties etc (there is one guy in my network who does this, and it's cringeworthy).
All the MN towers folk are on there, but I haven't had the courage to ask to link up with them... 
I tend to only accept a connection if I've met or spoken to them. I ignore or reject all those people who are just in it to get 500+ connections
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I also tend to ignore people who invite me with the bog standard invitation text! I think 'if you can't even be bothered to write a personal message..."
Let me know if you want to connect 