LinkedIn's purpose is basically the same as Facebook but on a professional level. The only benefit you will ever receive is if you use it and network, network, network.
Try creating a group and connect with your peers in your field. You will always gather new contacts and industry leads if you actually use it. I have seen too many people comment about how they have a page for LinkedIn or Facebook and they wonder if they should use it or it sits there. You aren't helping yourself by letting anything sit there.
My experience with LinkedIn has been good and I have even debated on creating a group in my field. However, be warned that creating a group is something you would need to manage. The plus side is networking and contacts.
So to out right answer your questions. Has LinkedIn worked for me? Yes, if I spend the time and network, communicate in groups, and search for company contacts in my field. Does my LinkedIn generate leads? I surely would hope so since I see company employed competitors and their HR representatives frequently visiting my page.
I know I may be speaking from an individual level but it still applies to a company level. All business owners have the same goal in mind, and that's to make money and be happy in what we do (or I would hope so). I hope this helps but as I've answered someone else's question about Facebook a few days back still applies here.
You have a free tool, use it, your results will only matter on the amount of effort you put in. Yes LinkedIn, Twitter, Facebook, and the like will always be annoying, but the effort and the purpose is clear. Attention!!