I flick between Opera, Firefox and Safari when I get suitably annoyed with one.
First off was Firefox, but when you've had it running for 2 weeks or so it's insane memory leak just brings the mac to a grinding halt, so that went in the bin to be replaced by Safari.
Which is OK except it has essentially 0 useful features, crashes pretty frequently, can't close tabs with middle mouse button, can't safe sessions without payware addon etc etc.
Then I moved onto Opera having used it on PC for years, and is still the most feature rich browser out of the box. It's only problem is it really doesn't render some pages correctly.
So now it's back to FF3 RC1 for a bit until I get annoyed with that.