I just got an invitation to Google+. After a brief time with it, I'm making a prediction: the Circles thing isn't going to work. With Circles, Google+ is making the play to become one network spanning many types of relationship and purpose by letting you restrict your sharing and filter your view of the global feed by sub-group. Work. Friends. People I follow. There are two reasons I doubt this will work.
Reason #1: I blogged a while ago about something we learned (the hard way) at TurnTo about granular privacy control. It doesn't work. There are people who will use your system, and there are people who won't use your system, but there are very few people who would-use-your-system-if-only-they-had-more-granular-privacy-control. Early on, we built a very similar privacy model to Google+: it provided groups to enable users to restrict what got shared with whom. Like Google+, we offered a set of starter groups and allowed user-customization.
Later, we ripped the whole thing out. We came to understand that (most) people want to manage privacy at the level of the network, not sub-groups within the network. People do their work-related networking on LinkedIn and their personal sharing on Facebook.
Twitter is great for following people you don't know personally (and therefor also useful for businesses to promote themselves). People decide who to connect to on each network based on what they plan to share (or read) there, and then they share fully with all their connections.
That's as granular as it gets. An item that isn't suitable for someone's whole audience on a particular network doesn't get restricted to a sub-group, it just doesn't get shared at all (or it gets shared with everyone with whatever consequences...).
Reason #2: Different system services are optimal for different types of network; there's no one-size-fits-all collection. As a professional network, LinkedIn provides a great structure for exhibiting your work history. As a personal network, Facebook has great photo sharing. As a network of mostly-nonpersonal-following, Twitter provides great link sharing.
The network services and the community co-evolve and specialize over time. If Google+ members really do try to maintain many different types of relationships within the system, they'll end up unhappy with the tools the system provides.
Either the tools will be too sparse, or the tools to support one type of network will be clutter to the others. (Would an elaborate resume system be appropriate for your Facebook profile?) In order for a social network to provide relevant services, there needs to be some level of focus to the type of relationships the network supports.
And once the network has that focus, groups become irrelevant.
If I'm right, someone buy me a beer. If Circles works out, the drinks on are me.