I know somebody who uses Flock on an ancient PBG4 because he says it's much faster and eats less memory than other Gecko-based browsers (i.e., not bloated at all). I looked at Flock, thought it was neat, but went back to my old browser quickly because I wasn't using any of the features.
If you don't use an application for organizing your pictures, the Flickr upload is a good idea, but for users of iPhoto/LightRoom/etc, there are plugins that export the metadata and make much more sense. Likewise with browsing my pictures, you could have just clicked on the pictures, because it was linked to the photo page on Flickr.
Flock just doesn't offer a lot of interesting functionality on top of what you can find in other programs - but for people who want to have it all in one program, it's probably a good choice.