I have it and have not yet received any real benefits - job offers, new contacts etc. through it. However, it only took a little while to copy info from my resume to linked in and set up a profile, and I figure at the very least, if someone on a hiring committee does a google search on my name when reviewing my application, it's good to have something professional like my linked in profile pop up in the search results.
As others have mentioned, the importance of a profile varies by field, and I get the impression it's of minimal importance in my field.