I'm signed up to it but don't really use it. It's a little like a social network site but for business people. It's a way of networking in the business world so you can make connections and get yourself 'out there'.
I don't pay anything to send people E mails, it's the free, probably limted version, although it does have it's uses none the less. I recently found an old school friend, an ex colleague and a former tutor.