My friend Tim Sullivan runs the www.sellinggeek.com podcast and he did a special on Linkdin that may give you some answers. http://tinyurl.com/5pzs6a.
In the company I just left, signing up for Linkedin was a sure sign that you were fed up and looking for another job <grin>.
I think it is most useful if you are looking for a job, or need to build a network that you can sell to, or to use to hunt for prospects. Its sort of like the online version of joining your local chamber of commerce. Heed Tims warnings and be careful what you share.