If Wikipedia, Skype and Flickr are the first kids on the Web 2.0 block, then Flock is very much the new kid on the block. It is certainly the buzzword in Europe at the moment. Some people like the tech pundit Robert Scoble are even calling it "Awesome"!
Flock's experience director, Bert Decram, and his co-developers are claiming that Flock is to web browsing what VHS was to TV, but I don't know if it is that big at all. Unless I am misunderstanding something Flock sounds like just another browser, except it has a lot of "web writing" stuff integrated. I have downloaded it and I must say that the integration is very slick. Blogging & photo sharing (for now) is very easy - you just drag and drop paragraphs and photos onto your blogs or photo sites. A little bit like Google's Picasa.
The idea is very good and it might help to force an IE rethink, but I'm not sure it is the "Office for the Web" (another Flock claim). Time will tell.
The guys developing this are all from the Mozillla foundation and I think Flock is based on Firefox. You can download a developer preview (version 0.49) at their developer page.