I've been using Flock for a week at work as my default browser and thought I'd wait awhile before commenting on it. As of today, I can declare that it is a great browser and a wicked idea.
It's Firefox and Safari combined and runs on Windows and Macs. About time! But the reason it really rocks is because of all the social browsing functionality it offers and the nice interface to it all.
As a Facebook user, it's fabulous access directly to your friends and in a more convenient way than Facebook provides. Try it for that if nothing else.
Now I'm just hanging out for my MacBook so I can install it on there and see how it plays.
Oh yeah, it's not as memory hungry as Firefox. It's fast!