Matt announced it here, and like a dutiful gnome, I followed his path:
We can’t stand spam. Who can? You have better things to do with your life than deal with the underbelly of the internet. Automattic Kismet (Akismet for short) is a collaborative effort to make comment and trackback spam a non-issue and restore innocence to blogging, so you never have to worry about spam again.
To use Akismet, I disabled SK2, though its not asked for. Thing is, Spam Karma 2 has been catching comment spam quite well, with about an average of 1 or 2 mislabeled-as-comment-spam mistakes, and about 1 or 2 comment-spam getting through, in a month (or two). Very good performance if you ask me, but based on what Akismet looks like, this appears to be a lot simpler, though not as customizable.
But of course I’ll dump it like a hot patatas when it underwhelms. Now, if only Matt can do something about referrer spam…