I don't plan on making any switch to it but at some point I'll probably sign up, just to see what it's like. It looks interesting from what I've seen.
It's unlikely to replace the monolith that Facebook has become, but stranger things have happened. Facebook dealt a similar blow to myspace, after all. It will mostly hinge on which people decide to join up and whether those people are perceived as cool. With the increasing numbers of older people on Facebook, perhaps that will become the place where we leave grandma and the parents to play Farmville while all the hip youngsters migrate to Google+.
Of course, it's also possible to have two popular social networking sites. Hyves is still dominant in the Netherlands, Hungary has iWiW, and I'm sure most countries have their own little version. That's a little different as it's language-based, of course, but the principle stands. I expect a lot of Dutchies use Hyves for their Dutch friends and Facebook for everyone else. If Google+ (or any social networking newcomer) can wedge itself in as the site for a particular segment of your networking, there's its spearhead. With the 'circles' feature, though, that doesn't seem to be what they're aiming for.