I've download Flock and indeed it goes down like...well, rather than putting in a naughty metaphor, let's just say it's not all that stable yet. But it is fun. The coolest thing about Flock is that it's integrated with del.icio.us, and it allows you to share bookmarks on the web.
You can see who's bookmarked your bookmarks, and see in turn what they've bookmarked.
It's like a blogosphere for the browser. It has a blogging tool integrated into the browser, and that's where crashes come in. Beware of "TopBar" for now.
But alas, it crashes. Save blogposts frequently. I was in the middle of a rather lengthy blogpost, and Boom! Flock crashed.
But get it and play with it. It's the latest heat on Internet Explorer, and that can't be all bad, can it?