What a great upgrade! 3D graphics in Croquet is noticeably faster and scrolling long web pages in Safari definitely is smoother and faster.
I used to be a big Linux fan (a Linux user since downloading Slackware over a 2400 baud modem over 10 years ago), but Mac OS X is such a better user experience (for coding, writing, web access) that I just use Linux on my servers now. Oh well.
It is even better that I hardly ever have to boot a Windows machine anymore (but I do keep one handy - work for some customers does occasionally require it).
I seems a little funny that I even care about operating systems: my favorite software (IntelliJ, LispWorks, Emacs, OpenOffice.org, FireFox, etc.) all runs well on OS X, Windows, and Linux. Still, using a Mac just seems so much nicer to me - really, no comparison.