Well I mentioned above that I like Flock, even though I do not use it.. I want to, because of the whole web 2.0 focus. It integrates a lot of the things that you need plugins for with Firefox. I like extendibility, I like a browser with the functionality that I need.. which is the reason why I do not use Safari.. It does browsing really really well.. and that's pretty much it
I have extensions installed for Firefox.. WebDeveloper extension, allows me to do a bunch of WebDeveloper stuff. I have MeasureIt installed so that I can measure elements on the webpage. I have weather in my status bar. I have stocks in my status bar. I have my adsense revenue in my status bar, I have Performancing installed so that I can access and write to my blog without actually logging in through the web interface to do so. I have EditCSS installed so that way I can do live updating to my CSS so that way I can tweak until I have it right and then copy my tweaks into the actual CSS file. I have Tab Mis Plus so that way if I accidentally closed a tab, I can unclose it. If my browser crashes or I quit it accidentally, I can open the last session. I like to be able to lock my tabs, to be able to move them around, to be able to do pretty much whatever I want with my tabs and not be limited. I have autofill installed so that way when I fill out forms I can have multiple profiles so that way I can enter contests for myself for my wife, for my grandma for whoever I have a profile for! I have delicious installed so that way I can manage my bookmarks online and allow my other computers access to those bookmarks. I have TargetAlert installed so that way I can view what each link I put my mouseover will do before I even click on it. I have Lynx Viewer installed so that way I can see what my websites will look like to those people with disabilities and have screen readers. I have JSView installed so that way I can view the CSS and JS files that a site uses so that way I can study a cool thing they did. Also themes.. I really like a nice clean theme.. I do not like to be limited in the way my browser looks.
Now Flock does all those things since I can use Firefox extensions.. plus it's faster, plus it has some of those things built in so that I do not need to have an extension for.
It has built in access to almost all photo sharing sites out there.
It has built in access to your blog.
It has built in access for online bookmarking.
It has built in Bit Torrent.
Built in RSS feeds
And a whole lot more.. it's an extremely nice browser for those people that are into online communities and some serious functionality in a browser. Again the only reason I do not use it yet is because Tab Mix Plus does not work on it and I really like the ability to be able to do whatever I want with my tabs and not be limited.
Practically none of that stuff that I like is in other browsers or some of it is, but not all of it. So I stick with Firefox and wish I was using Flock
Like dtravis7 said.. it's very very subjective, some people just want to browse websites and that's it.. if I was that way I would definitely use Safari. Opera is nice, but has problems in rendering and since Im a web developer.. I can't have that and since majority of people use IE then Firefox, then Safari.. I need to make sure my applications work on those browsers.. Flock uses the Firefox rendering engine.. so it's covered. It definitely depends on what you are going for... thus the reason why I asked