I use Firefox almost exclusively. I used to be a huge fan of Opera but their lack of extensions for even simple things like reliable ad blocking really hampers the experience. I recently spent a week on Chrome and while in general it was fine, there was one glaring issue that I couldn't get over: The complete lack of ability to manage plugins (like flash, adobe reader, etc.). I hate when PDF files load "in" my browser, I'd rather they download and then open in my Reader app. There's no easy way to get rid of this behavior (you have to delete the .dll plugin, which deletes it for everyone using the box, or get a hacky 3rd-party extension .bat file not even published on the offical chrome extensions site). People at google are saying this will be fixed at some point, but from the bug reports, it's been over a year and nothing yet.
Chrome is cool because it's fast -- the interface is faster than firefox for sure. But not worth it for me in the end.