I have been using it for about 4 years now and find it very useful for staying in touch with people I've worked with and networking to find people with common interests, etc.
I use it. I got back in touch with a few classmates, even one that is 8 time zones away. I also used it to hook up with some of the big name in the webmaster world.
I have an account but I wouldn't go so far to say that I "use it". I suppose I should find my info and see if I can log in!
I absolutely love Linkedin. I have done business through it and has helped me keep in touch with all my contacts.