Joined due to a decision by my grad school program to use LinkedIn to network alumni. Works better than our alma mater's system. We're a special breed, with less than 1K alumni total (only 50-60 grads/year). In just a few months, we've gotten almost everyone online, and we have our own "group" on LinkedIn.

Otherwise, I've used it to reconnect with former co-workers & classmates from undergrad, and to try to network with another limited population to which I belong (awardees of a particular Fellowship).

I've connected with others sparingly, preferring those I know, or at least those to whom a rational professional connection makes sense. That said, it has lead to useful business leads for my consulting business, and to being recruited for some very interesting opportunities that I would not have otherwise heard or thought of.

I've got just under 200 connections, and really view those with boatloads of connections with a lot of suspicion.