Safari is growing in popularity, according to recent figures, with estimates putting the browser as breaking a 7% share. That may seem like a small figure, but it truly wasn't that long ago when even Firefox was below that mark. As the biggest “alternative” browser, Firefox is continuing to grow as well. Safari's growth is coming primarily at the expense of Internet Explorer, which has now dipped below 70% for the first time in many years, falling far from the peak of 95% it held in 2002.
Safari goes hand in hand with Mac OS X, so watching growth of that browser is a good indicator of Mac growth as well. It's not entirely accurate, as you certainly aren't forced to use Safari on a Mac and Safari is available on Windows as well. The small discrepancy between the amount of Mac OS X users and Safari users can likely be attributed to people using other browsers on the Mac, but it still goes to show that the browser is doing well.