As an executive search consultant I use Linkedin all the time and pay a monthly subscription for the privilege. I only contact individuals who flag that they are interested in career opportunties, but have filled 3 senior searches in the last 3 months with candidates from Linkedin.
I am not sure if the founders intended it to be used as an open recruitment database, but I find it invaluable - it is particularly useful for searches in North America where, as far as I can tell, everyone uses it.
If you are job hunting then I would suggest being on there as you never know what might come through - other than that I'm not entirely sure what benefit can be derived from it. I have found that people are far more happy to give a peer to peer recommendation to me on linkedin than you might think, despite a fairly tenous link with me - perhaps because the brand of linkedin has developed a good sense of trust around it. This could also be very useful for proactive direct job hunting - rather than just the 'middle man' stuff that I do.