I think XKCD put it best:
Google+ is tempting even to someone like me who never uses any of the social networking sites anymore. I’ll admit it, there was a time I was on MySpace quite a bit. I closed up shop and moved to Facebook when most of the world did, but that was around the time I lost interest.
But Google+ appeals to me because by setting up my social map in Circles, I can convince it that my co-workers are not necessarily my friends and are certainly not my family. That seems to be their killer feature, and it’s a good one.
But it’s also the feature, as Dave Winer points out, that’s going to be the first to break down. Dave likes organization, and he likes building tools. He describes several failed attempts at creating tools and - more importantly - habits for tagging and curating his tons and tons of written thoughts for use in creating a book some day. The pattern every time seemed to be: create new tool > get excited about tool > use the tool a bunch for 1 to a few days > quit using tool. That pattern certainly seems familiar to me.
So apply that to Circles. You might feel a rush to organize your friends into categories when you start to use it. But you’ll give up after a dozen or so, as soon as you hit one that defies categorization. You’ll say to yourself “I’ll come back to this later.” You won’t.