Received the invitation to test the Flock Developer Preview. Cute, but based on Firefox and XUL (hence heavier, clunkier and less polished than a native application on OS X, or any OS actually) and… without much of a point. It’s supposed to be a “social browser”, but all it does is display Flickr photos in a bar (but only one user’s photos) or managing bookmarks on del.icio.us (but, there again, without any consideration for the community aspect of it — it’s only saving and reading your bookmarks on a remote server, is all). Ah, and a blog editor, but I have no use for that since my blog isn’t compatible with standard APIs.
Considering the beta’s maturity, I doubt it’s going to get much more useful by the time it’s publicly released. There are much more interesting and much more social Firefox extensions already.