LinkedIn has been a very valuable and useful tool for me. When I give sales training, I always advise sales people to research who they are meeting with in order to customise their messaging appropriately. You'd be surprised how many sales people don't do this and are so much more unprepared for meetings.
It has also been a great way to connect with old colleagues and clients as well as get feedback. I've never been someone who 'blows my own horn' or done a lot of personal PR. Mostly because I just prefer to get on with the work at hand since it pays the rent a lot better. ;-) I do acknowledge the need for some personal PR (I get told this often) so the recommendations are my way of assuring those who are considering hiring me of what they can expect. I also appreciate the feedback so much from clients, since as a consultant, you don't really have a formal evaluation process.
The groups in LinkedIn are also really taking off, and participating in groups is a way of building new connections around similar business interests and passions. From one of these groups, I recommend reading Paul Castain's Sales Playbook post on LinkedIn if you're interested in getting the most out of this valuable social networking tool.