I use Opera probably 95% of the time. Features such as Speed Dial (without the pages moving around a la Chrome/Safari, and with shortcuts), being able to easily add search engines to the address bar (and Firefox has the gall to call theirs awesome) linked bookmarks/Speed Dial/address bar search engines over the Internet between any PC you log onto (ideally also with Opera), configurably shortcuts, and general speediness are the difference. Really haven't figured out why other browsers don't allow you to easily customize the shortcuts. Just because they think they have the best shortcuts doesn't mean they do.
I also have IE8 and Firefox 2 and 3 installed. For compatibility's sake. I never find myself missing their features. I've tried Safari 3/4, Chrome, and Firefox 3.2 Alpha on Windows, though I no longer have any of them installed. Safari 3 was the worst, and should have remained on Mac, Safari 4 was okay, Chrome was too minimalistic, and Firefox 3.2 was the fastest browser I've used yet (but I don't trust Firefox alphas much in terms of security). Firefox 3.2, by the way, is the version that is due to come after 3.5, and is now being called 3.6. Not sure if it's still so zippy or if additions to it have slowed it down some.