I find G+ especially useful in a "semi-professional" use case. G+ is just an extension to gmail that adds some easy ways to organize work groups.
For example, a meaningful percentage of my gmail activity is sharing links and info with project teams. It's more intuitive to me to create a circle for each team and post to them via G+ than it is to prepare an ad hoc email to them each time. Also, I'd rather organize that conversation as a private stream with discussion, etc. It avoids clutter in my gmail inbox.
I also spend a lot of time chatting and talking on Skype. Now that I can organize group video chat via G+, I can cut Skype out of my workflow. I don't like using the Skype client at all.
I realize that solutions to these use cases existed before G+, but G+ provides a unified interface that has a powerful effect on my willingness to organize a graph inside Google.
I can see Google+ taking off for enterprise users of Google Apps, once it's released to them.