Useful website. I used it more when I was a headhunter. Obviously it's a fast way to find who works where in what position to "poach" them.
Try not to add too many groups - the updates and emails get really troublesome - of course you could turn them off.
Connecting with old colleagues and classmates in this network is better than FB IMHO because you keep it "strictly business" and you never know when that contact might become useful (I know it's not right to view people like these, but they will tap your network as much as you tap theirs).
When I moved out of headhunting I talked with HR professionals I knew during my career to secure my current job. There are thousands of people that will apply to a job with "dear sir/madam" or "to whom it may concern" etc. The impact of writing DIRECTLY to someone makes a difference, via email, personal, direct - LinkedIn helps you find the person to do that.