My use of LinkedIn is very limited. I used to use it quite a bit, but I got tired of it. It isn't really geared for smaller businesses or consulting work, but that is just my opinion.
I think LI is best used for recruiters and job hunters. It is like a powered up online resume with additional profile info. For recruiters and headhunters, LI is like a goldmine for them. They have access to a ton and I mean a ton of information on professionals everywhere and it is easily searchable. Perfect for seearching on skillsets and job descriptions to find candidates for jobs, even if that person isn't even looking for a job. (This has happened to me before) They can also leverage the extensive networking component and look for candidates through the people in their network.
For a job hunter - it offers all of the same benefits as a recruiter, just reversed. If you fill out your LI profile to the max and optimize it for keywords just like you would for your website - I guarantee in no time you will have recruiters contacting you for positions they think are a fit for you.
Other than that, I think the last main benefit is using it to make connections for biz dev partnerships. You can find contacts across organizations and leap frog your way into getting introduced and start discussions for potential partnerships. They do have the whole 'Ansers' section which is somewhat useful, but not very intuitive for researching.
Overall, I think the site is pretty useful, but just for specific purposes. I cannot see myself returning to it consistently, but only when I am in need of beefing up my resume and looking for new work.