For me Mac OS X is a pretty decent OS - and certainly far nicer than any version of Windows.
Having a proper CLI underneath, of the Unix variety, is really nice. The desktop software feels really polished as well. In addition the eye candy is far more muted than the earlier Mac OS X editions - smooth greys, etc - so it isn't distracting, unlike the eye candy on Windows Vista and Windows 7. In addition the 'effects' - Expose, etc, are genuinely useful and usable, unlike the effects in Windows 7.
I also use Ubuntu, and the latest version is quite nice, and finally has a decent OS font. However the last automatic upgrade hosed my wireless networking configuration, broke the keyboard and trackpad (requiring me to plug in USB variants) and set the keyboard back to USA from UK, and when things break in Linux, it's a PITA. However it's far better than OpenSUSE which I have on another computer.
Mac OS X gets out of my way and the UI doesn't halt the system when it is thinking, and that's what an OS should do. In addition, when I need to probe or tweak the preferences, it's easy to do and logical. Mac OS X Lion has a couple of new features, one of which is very much like Amiga screens too.
The Finder isn't the greatest thing ever though.
Also the hardware is rather nice.
