LinkedIn has become most notable as a talent pool resource and used a lot by corporate recruiters. I use it like an online rolodex of different experts that I might want to go to and also to learn more about someone I've met through networking. Like any social media outlet, it is only as good as how actively people engage through it. I don't really follow a lot of groups on it, myself. I put a bit more into Google+ and Google Places for Business when I have the time. If you put your company into the system as well, be aware that the job posting option is big bucks (there is a workaround) and probably not in most people's budget to use for recruiting new team members.
Like Sam says, it is another way of giving the search engines a way to verify that you must be "da Man" (or "da Woman") in your field. If you're going to do it, do it with real gusto and put a lot of content in your profile about your business, its size, your specialty if you have one, and have someone proofread it for spelling and grammar. Don't be shy about any community things you do for Arbor Day, other projects like helping with parks and playgrounds. It is really your online resume.
