I never used it. The only reason I had a profile was that random people started sending me requests. Then I sent a few to others, but always on a whim when I had a few minutes to look around and see connections they recommended. Then I saw my girlfriend using it like crazy -- she is really good at relationships in general. She connected to a lot of people close to her, and through them, they started making introductions to people they were connected to who she wanted to get to. It has paid some VERY SERIOUS dividends for her.
My profile is only so full. And I don't have a ton of contacts. But I recently did a few things. I belong to a few groups, and one for where I went to school had a thread saying, "If you started a business, tell about it." I figured what the heck, and posted something brief with a link to some work samples, and that day I heard from half a dozen people I didn't know. Half of them wanted introductions to others from me (which I was happy to do for two of them), but you never know where those kinds of conversations might lead.
I also recently contacted a few people I don't talk to that often (who I had business ties to years ago) who might have ideas for reaching a new business segment I would love to expand into, and even though that kind of "marketing" has never been my forte, the interactions have been surprisingly good. A couple of people have already put me in touch with people more directly related to what I am trying to get to, and even though none of it might go anywhere, I am finding it to be an interesting resource.
I am usually dubious -- kind of cynical and try to be self-reliant -- the guy who never has to ask people for help! But face it, that isn't possible in this world anymore. I have had steady clients that fed me work for years who have had their budgets slashed and that means having to solicit work rather than waiting for it to come to me. It's difficult, but LinkedIn is a definite resource.