There are lots of discussion about Google+ in the blogosphere already. If you are curious nothing beats trying it out yourself. So if you have an invite to Google+, just use it and form your own opinion. I will just note down my observations and impressions here.
First, it looks like Facebook somewhat, but it isn’t Facebook. Like XKCD says. It has profiles, status updates, photo uploads and tagging, and messages. So it is similar to Facebook in many ways. But it is also fundamentally different.
The main difference is the relationship model. Google+ has asymmetrical model. You don’t friend people there.You don’t need to be anybody’s friend. You include them in your circles, and they don’t need to approve it. There are several predefined circles: family, friends, acquaintances, following. This breaks the friendship model that introduced by Friendster and continued by Facebook.
When you include people in your circle their public updates will appear on the status stream. If they choose to include you in their circles, additional updates may appear as well.
Actually Google+ is more similar to Twitter in this regard: you can follow someone but you cannot insist that he/she follow you back. Except you can write more than 140 characters on, photo sharing is included, and you have better view of discussions. So you can see Google+ as Twitter on steroids.
This asymmetrical treatment also solves the problem faced by any social network based on friendships. Perhaps you cannot follow your friends because they are not yet in Google+ (or Twitter), but you can just add interesting people that already there in your circles.
There are two more main features you can’t find on Facebook (or Twitter for that matter): Sparks and Hangout. Sparks is meant to be “a constant feed of things that you’re really into and share it with friends” but I also see this as a good way to compensate the lack of Google+ buttons for sharing links on websites (compared to Twitter and Facebook buttons that have already spread everywhere).
Due to lack of bandwidth I haven’t really try Google Hangout yet, but there are already lots of good words about it. It is said to threaten to displace Skype for videoconferencing. Because I don’t have Android phone, I can’t comment about the Google+ client for phones. There is web app for Google+, but I only can see the basic XHTML version from my Nokia E63. Is this a sign for me to buy a new phone?