I used to use Firefox, but it seems to have become very bloated and kinda buggy lately (but that might just be the Mac version), so I moved to Safari. It's a massive memory hog (it can use up to 800MB of ram, which might just be a little too much...), but it's fast, does what I need it to do, has some good Mac-integration and ClickToFlash <3, it looks so much more elegant than any flashblocker I've found for Firefox.
Firefox also seems to be slowing down in terms of development, it doesn't get 100 on Acid3, its HTML5-support seems to be behind Chrome and Safari, and in pretty much all speed tests it only seems to beat IE (which isn't much of an accomplishment). I used to love it, but it's really going downhill, sadly. It still has a place on my dock, though, for those times when I need a specific add-on.
I did waste quite some time looking for a decent adblocker for Safari, as well as a way to configure some things I had gotten used to using Firefox (Glims seems to do the trick for most of these), and I still haven't found a decent video-download add-on (Window -> Activity does the trick for now), but I don't regret changing.
I don't really like Chrome's interface, tabs-on-top just feels kinda weird to me. I don't really see a reason to start using it over Safari, but it's not a bad browser by any means. It uses Webkit anyway, so the more people use it the more people should start working on Webkit, which should also help me as a Safari user
Opera just feels kinda iffy to me. It is by no means a terrible browser, and I applaud their recent efforts to join the ranks of Safari and Chrome in terms of speed, but I don't see myself switching to it any time soon.