Google has done the "invite only" thing on multiple product launches now. It hasn't always (or even often) resulted in a long-term success, in some cases I would argue it even hurt adoption, but it's generally a good strategy I do agree.
I think if G+ were, say, like Buzz, or something else equally unimpressive and poorly executed, then it would not have the interest it does. It is exciting because it is truly and legitimately a better system, despite its faults and bugs (for being out less than a month, it has surprisingly few actually). Google has, in my opinion (and evidently in the opinion of a lot of the current users) "gotten it right". I can think of no better reason for a product/service to succeed than by just doing things better.
Sure, Google is leveraging existing systems and services (and their corresponding existing membership and content). So this is not necessarily something a random tech startup could do (at least not as well). Still, it's the underlying cleverness and usefulness of the ideas (Circles, especially) that are making the product shine right now. The UI is great too, but admittedly needs some bug fixes and polish. Rather than complain about it (or in addition to doing so), people should submit bug reports through the also well implemented bug report tool.
Google is iterating remarkably fast here. We'll see company/interest/etc. pages soon, apps also (hopefully with a better way to manage them than Facebook has), and likely every other important feature FB has. They're not hard to implement, and Google has already shown with what we have in G+ *today* that they can, in fact, implement a slick, intelligent system with a high level of features, and still maintain a fairly intuitive UI.