I haven't used it (though I haven't used Facebook either), but it strikes me as a gesture by Google towards its shareholders/market analysts as much as to its users. It needs to be seen to have a stake in the social networking market, for the (ugh) 'completeness' of its brand. A decent parallel would be Microsoft's creation of Bing - they will never usurp Google's dominance of the search engine market, but it shows they have a broader portfolio of products (ugh 2) and are at least trying to compete, and that satisfies various bean-counters and decision-makers in the industry.
Perhaps a more illustrative analogy would be to compare Facebook to the Nintendo Wii - neither were the first in their respective fields, but both managed to hit upon something unique and draw in loads of new people who had never previously used a social network or played a videogame. Other companies see this innovation and fancy a portion of this new revenue stream. Hence Microsoft and Sony creating new motion-control videogame things and Google entering social networking. If you can tempt even 10% of users away from Facebook/Wii, then that's a pretty big new revenue stream.
Have Kinect and Playstation Move overtaken the Wii? No. Will Google+ overtake Facebook? Well... Facebook completely stomped on myspace, which in turn ruined Friends Reunited, so the precedent does exist. Bear in mind also that Google have tremendous brand loyalty and recognition. I would wager that near 100% of Dissensians use Google for searches - you just do, don't you? - and Chrome and Gmail are also (as far as I can see from limited testimony from friends, etc) well-liked. The amount of people clamouring to join in the Google+ beta indicates that it will be a success, and the unification of browser, email and social network under one umbrella is an appealing concept for many people - you only have to remember one password, for a start. I can't see it eclipsing Facebook overnight, but Google have the finances and resources to make it a strong rival, if not a dominant one. I think if it can be proved to be demonstrably superior (though, as stated, I've not used either so I don't know quite how - more features, more joined-up functions?) then there will be a steady flow of people looking to 'upgrade' - in the same way that DVD gradually usurped video.
I'm still not interested in signing up, though, social networking terrifies me. I'm happy enough with the few real-life friends I have and would rather speak to them on the electric telephone than look at pictures of their holidays or read about what they're having for dinner.