There have been a couple of interesting posts recently about how Google+ (G+) could be a threat to Facebook, Twitter, LinkedIn and Skype among others. I think there is significant inertia in these platforms in terms of their established and reasonably happy userbases, if only because these users themselves were not acquired in a year or two. However, G+ does have long term features going for it, such as Google integration and the ability to adapt to various use cases.
Apart from the early-adopters, the first real chunks of users might come from niche features done right. An obvious example is videoconferencing. The G+ team has done an amazing job with Hangouts and people (and businesses?) will find this extremely convenient.
Another case I see for it is blogging. G+ makes it really easy to share your thoughts. There is clearly enough technology in there to satisfy most bloggers' needs. You also get immediate exposure, integrated comments and best of all, sharing your content is a click of a button away.
Can we use it today? To a certain extent, yes, and in fact you could consider anyone who is writing longish pieces of writing in G+ to be using it as a blogging platform. We do have to wait a bit more to see how Google will handle the publicity of your posts. Ideally, you would like them to be visible to people who are not using G+ in a nice way. Another issue is lack of styling options. There is a conflict with Google's accessibility standards and most likely you will not be able to enforce a particular style to your writing.
That said, most people using, e.g., posterous use the same template, and G+ might well end up eliminating the need to choose a blogging platform.