Interesting point of comparison:

When Google made the transition from the "old" GMail interface to the "new" GMail interface, the changes were quite subtle, and they had a large chunk of time where you could "opt in" to the new interface, then they threw the big switch and let you "opt out" of it. In the end, I don't think there was much screaming about that transition.

For Buzz, Google's being somewhat less ceremonious about it, but then it's pretty easy to ignore buzz and just read your email as normal. If I were to add you to a buzz thread, perhaps by referencing you with their @-notation, then the thread would show up in your inbox like any other message, which you could then read and respond to. Or, you could "archive" or "delete" the thread and you'd never see it again.

Cleverly, all of this still kinda works via IMAP, at least from a read-only perspective. When the thread updates, the message bumps back up near the top of your message order. You can't use email replies, unlike FriendFeed and more recently Facebook, however you can hit the link at the bottom of the message which takes you to www.google.com/buzz/......

I actually think Buzz works just fine, but then I've drunk the Google Kool-Aid pretty thoroughly. I now find myself wondering when/if they'll integrate Google Wave in the same way they integrated Google Buzz. Or, maybe Google will provide third-party plugins so other parties (Twitter, Facebook, etc.) can integrate with Gmail the same way that Google Buzz does.

How, exactly, this sort of cross-service integration is ultimately going to be supposed (public-subscribe services? app plugins? third-party reflectors?) is going to be one of the big battles over the next few years.