Prior to becoming a Mac user I had a deep hatred for Safari. Why you ask? Safari was the bane of my existence as a web site developer. Since I had a Windows centered view on the world I built everything with IE and then Firefox in mind on the Windows platform. We would spend huge amounts of time getting a dynamic web site to work properly and then someone would load it up on a Mac in Safari and it would just fall apart. Things didn't line up, pages that used DHTML and more advanced presentation capabilities would simply not work. It was a mess.
The only way we could get our web site to work on Macs was to request that people that wanted to use our applications just use Firefox on the Mac. It was very consistent with the Windows version and since it was free we figured it was a reasonable compromise.
When I started using my Mac I figured I would immediately use Firefox for everything; it was one of the first applications I downloaded. I kept Safari as my default web browser though and started to play with it out of curiosity. Nearly every web site (with a couple of minor exceptions) worked fine. And I noticed something else too; Safari was fast - really fast.
Having not followed Safari closely I can only assume that Apple has made significant improvements in the rendering engine for Safari, both in terms of performance and HTML rendering. Safari looks great from a UI standpoint, has tabs and all of the shortcuts I'm used to on Firefox.
What Safari doesn't have that Firefox does is a huge range of add-ons that really add value to the web browsing experience. There are plug-ins for anything you can think of. Firefox is nearly a small operating system of its own.
Did I mention that Safari is fast? Well, Apple has a public beta for the next version of Safari and it's supposed to be double the speed of the current version. I've played with it and it does seem even faster, however it's still in beta and I've found a couple little glitches here and there. The nice thing is you can install it along side Safari and they peacefully coexist.
Firefox is also making some big improvements with version 3 - and their beta is also available to the public, though I haven't installed it yet.
Even when I was on the Windows platform I always had two browsers handy; Firefox and IE. On Mac it's the same way but I find myself spending virtually all of my time in Safari and whip out Firefox when I don't have a choice.