I was contacted through Linkedin just last week for a position that would be an absolute perfect fit, except for the city it was located in. I have only been in Chicago for 2 years, and relocation is simply not an option.
Admittedly, I also get far more contacts offering 100% commission sales jobs, which gets annoying.
A relative of my partner works as a sort of headhunter, and LinkedIn is her primary resource.
Linked In is, not unsurprisingly, huge in IT.
Its also a good place to do research. You know you are interviewing with Curt Russell at Acme, you look him up on Linked In before your interview. Now you know a few things about Curt.