I remember years ago when I first got on Facebook. Zero of my friends were there. There was almost nothing to do on there other then talk to myself. But I invited a few of my friends that were on MySpace. Each one asked me what the point was, why they would do MySpace and Facebook, when all of their friends were already on MySpace. I didn't have a good answer to that other then the fact that so many MySpace pages were truly ugly. Facebook just seemed like a grown up MySpace to me.
As friends started getting on Facebook, it became clear that it was a much better way of interacting then MySpace was. But as my Facebook friend list has grown, I find myself censoring what I post there. If find myself asking whether a post is appropriate to post now that my parents, in laws, grandparents, coworkers, and neighbor's kids are on Facebook. I can't have frank discussions about adult topics because of the kids, I can't brag about job interviews because of coworkers, and I'm less likely to talk about parties because of the grandparents.
Circles solve all of that for me, if (and these is a big ifs):
* All of my Facebook friends join.
* I actually take the time to organize my friends, coworkers, etc.
Getting all of my Facebook friends, or at least a critical mass, should be relatively easy once we can invite people. But I was a huge Wave fan, but frequently didn't have anyone to talk with there. Most of my non-nerd friends just didn't get it. Google+ is much simpler to understand than Wave.
Organizing my friends isn't something that I'm particularly worried about for myself, since I tend to be a little OCD about that. But I am very annoyed that I can't import my Gmail groups, since I've already basically organized everyone I know into little silos: hockey friends, Googlers, current coworkers, Texas Friends, Virginia Friends, etc. I have to completely redo the work that I've already done. I imagine that I'm in a small minority of Google+ users that have their Gmail contacts so carefully organized.
One thing that seems to be missing with circles is circling up circles. For example, if I have a circle for Texas Friends and one for California Friends, I should be able to have both of those circles in a bigger circle of Friends.