Just recently updated my Mac from Leopard to Snow Leopard (OS X version 10.5 to version 10.6) and am quite happy with the bump in speed. Although the OS boots by default into a 32-bit kernel (it is possible to boot into a 64-bit kernel for servers and specialized desktops), it is now able to run 64-bit apps. This apparently makes it possible for apps to address the vastly larger memory spaces that 64-bits implies. Two of the most important to me are the Finder and the Safari web browser and they are both now 64-bit apps. The speed of Safari is impressive and I briefly used Safari as my default browser. However... I always come back to Firefox. On Windows, I quickly tire of IE. And on Linux, there is only Firefox, practically speaking.
Why is that? For me, the plug-in capability is important. Zotero is designed to work only with Firefox and, what with my recently acquired Evernote addiction for genealogy work, the Firefox Evernote web clipper is much more convenient that dragging the "Clip to Evernote" link to the Safari link bar. (Edit 10 September 2009: And Keywords are useful too.) So, although Firefox 3.5 is noticeably slower to render complex web pages that Safari, it's my browser of choice.
It might also be worthwhile to note that Google has been the primary source of funds for Mozilla. But Google now competes with Mozilla with their Chrome browser and I worry about whether they'll soon cut off funding for Mozilla. Other than a beta version, there is no Chrome for Mac so I haven't tried it yet. And I don't know if it has a plug-in capability like that of Firefox. But even if it does, I also have seen zero mention of plug-in development for Chrome.
Parenthetically, I was silently skeptical about seeing any increase in speed when moving from 32-bit Mac OS to the 64-bit variety. I remember back to 1995 when Windows switched from 16-bit (Windows 3.1) to 32-bit (Windows 95) and Microsoft and PC Magazine pundits seemed to imply that there would be a huge increase in speed. Alas, I saw no such thing. There are many who would say the reason is Microsoft OS and apps bloat.