I would agree that it’s more useful in certain circles than in others. However, if you’re looking for a job or for freelance clients/connections then it’s a very useful resource. I get several freelance/contract inquiries every month via linkedIn, and there are a LOT of recruiters on there looking for designers/developers and advertising open positions for web-related jobs.
Here’s an example of how it’s helped me recently: 3 months ago I wasn’t actively looking for a new job but was approached by a recruiter on linkedIn who sent me information for a front-end dev job at a large online university. I ended up getting the job – it was a great position and paid 50% more than I was making at my current job. After 3 months I was made an offer by my boss at my old job for a promotion and pay-rise to go back, which I took. That never would have happened if I wasn’t on linkedIn.
I mean where else can you have access to hundreds/thousands of professional connections which you can use to advance your career in various ways? I’d say that if you know how to use it effectively then it can be a very useful tool.