There are many browsers out there: IE6, IE7, IE8, FF, Opera, Safari, Chrome.
Then there are tons of mods for each of those. On the mac, a few lesser options (FF, Opera, Safari, soon to be Chrome). I used to scoff at Safari users when it was around the 2.0 release (even 3.0 seemed crap to me). When I adopted the Mac, however, Safari fell into a different light for me. The number one reason it did was because of it's speed.
In the 4.0 release, the first thing you realize about Safari is how fast it is. While non-web developers would often remark that there isn't much of a difference, it's something you pick up very noticeably when you spend 10 hours in a browser. It's not that every second counts, it's just that you notice it and lean towards the one that will save you any time at all.
As a developer, it misses many things like Firebug, plugins (has a few, but not comparable), web developer toolbar, and while Safari has it's own web-kit based inspector, in my opinion, it's terrible. You can view the DOM, but not do much with it.
I find that the only time I'm not using Safari is when I need to do some post-render editing using firebug, or test out a plugin in FF; any other time, Safari and it's speed an UI dominate for me. My only hope that it gets a little more plugin compliant to make it more useful more of the time.