Personally I do not think it will be the Facebook killer everyone seems to think it will be. I do not know anyone who is a frequent facebook user and not otherwise technologically literate who has heard of it or has any real interest in it, whilst Google are really pushing Plus even with their "limited" invite system i'm not sure it will really take off.
The way they are handling it has to be done carefully too, whilst someone are clambering all over the place for an exclusive invite from someone already in others have quickly lost interest in waiting to join in via the know-someone-who-knows-someone trickle. At the moment there are features in Google+ that feel like those available on Facebook, but they've been thrust into the users eye rather than sidling in via an update no many people seem to read.
It is nicely designed, but I expect no less from the Google brand. Until the people I need to contact via social networking migrate to it though it has nothing that wows me. I've said it before for those with an appreciation of good design and excellent coding it's delicious, for everyone else it's more of a "why do I need to?" thing.
I'm also wary due to the eerie similarity to Googles other attempts at social networking, they were praised as being the facebook killers, the next innovative means of socialising on the net, but they quickly drowned due to inadequate handling by the Google team when it came to access. Although Google+ has already surpassed its ancestors and some people have full networks i'm still sitting there thinking "this is it, this is what people were gushing about?".
Currently I do believe they will co-exist fairly well, each suiting a different type of social networker. It did not shock me to see who from my friendslist has already abandoned Facebook in favour of Plus, they are the ones who use it for slightly different reasons to the rest. I go on to communicate with relatives who barely understand how Skype works and find it amazing they can play games online where they can feed fish/build cities/fight dragons. It'll be a while before they move over, if ever.
We'll see. My opinion may change in the next month or so.