For well over a year now I've been using the LinkedIn
business networking tool. It's more than simply an online address book or contact list, but that's basically what it is. I find it useful to help organize all of my "important" business and professional contacts, and it gives you access to their detailed profile information without having to key any of that information in yourself. It also lets you see what contacts your personal contacts have. You can also search for names or company names or by keywords and it will help identify which of your contacts provide the best "path" to a desired contact.
To date, I have accumulated 59 "connections" in my personal LinkedIn network, but I haven't gotten any really big-bang benefits from it, but I do find it at least marginally useful.
I just attended a conference on private equity her in New York City and came away with twenty business cards, eight of which were already in LinkedIn and who I invited to be "connected" to me. I added another six as new "connections". I'm still considering what to do with the other six, but I may "invite" them as well.
I'd welcome comments from readers as to their experiences with getting any value out of LinkedIn. And of course, you may invite me into your own personal LinkedIn networks.