It's only limited to Gmail users as far as I am aware, it's pretty boring though.): I thought it'd be so much better...
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Buzz is still in beta version, I suppose. When you open your gmail, you can find it under the inbox link. It's actually nothing great, but it's a bit fun though.
I don't get it the same way I don't get twitter, I just don't get the idea behind it (twitter) and the point of putting minute by minute updates on there.