Buzz allows you to post items - links, images, videos etc to a wall within your gmail, that all contacts following you can see and comment on, much like the facebook wall. Buzz integrates well with Google Reader so that when I 'like' an item on Reader, it automatically appears in my Buzz like it does on my reading list feed. Of course Google Reader has to be added to my list of "connected sites" within Buzz. Connecting sites is easy and Google Buzz will offer to link your twitter and blogger accounts too - coool.
My biggest problem with Buzz is privacy - the default settings are just too slack that you can virtually follow anyone and easily find their email address. By default, you can view the profiles (Google Profiles) of people who follow you and those you follow. You can also see all their followers and those they follow and so on. What's worse their Buzz posts are also displayed on their profiles by default (I'm sure this can be turned off). I found this a bit disturbing. Of course when posting one can select to make posts private - only viewed by certain contacts but the tick will go unnoticed for most people.
I think the thinking behind Buzz posts is closer to tweeting than the facebook wall but it is definitely sitting somewhere between.
Comments on posts made by people we follow can also create unnecessary noise as they also increase the count on unread Buzz posts. What's worse, if you comment on someone's post, all comments made after yours will also appear in your inbox like ordinary direct email. To be fair though, there is an option to 'mute' a post - which makes it dissappear and stop subsequent comments from comming to you, much like Google Wave.
Another annoyance is that since my Reader account is "connected", anything I 'like' is by default shared public. I 'like' so many things in a day I can only feel sorry for the people who follow me. The Buzz inbox can soon be cluttered if one follows too many people. This is annoying because it is within email and unlike facebook feeds, we normally want to check on any new item in our email box.
It would be nicer if you could specify within Reader if with each 'liked' item, you wanted it also posted to Buzz. Linkedin does that well. When you link a twitter account, you can specify that it doesn't grab all tweets. If you want your tweet to appear on your linkedin profile, you prefix it with #in. I'm not sure if Buzz even supports the '#'.
Buzz is a great innovation but Google still needs to do some work there. I would like to see more twitter like features like "retweeting" and the ability to reply to a specific person with the @mashtales. This probaly needs Google to give us handles that do not reveal our email addresses.
Should Facebook and Twitter be afraid, maybe not very but this latest kid on the social networking block carries a brand with it and also rides on the millions that long fell in love with Gmail. No doubt Google has some more feature to roll out on Buzz, we will just wait and see.