I had the opportunity to sample Mac OS X 10.5.1 (and 10.5.2) recently. Here's some random observations and conclusions. YMMV.

• Apple is obsessed with UI and it shows (in a good way). The 'fit and finish' is very nice.
• I don't like its limiting of window resize handles to the lower-right corner. I don't want to move the mouse that far, when it is near the top-left corner of a window. NADBBA. (Not A Deal-Breaker, But Annoying.)
• I miss using Alt-letter to open top-level menu items. Yes, you can get there (eventually) by pressing Ctrl-F2, then selecting the menu via tab (or first-letter if unique). Yes, Mac has a shortcut for items within the window. No, I can't remember them all. The underline Alt-letter convention of Windows and several X-Windows window managers means I don't have to memorize 843 alt-control-shift-letter combinations. NADBBA.
• I'm a 98% keyboard user. Yes, it is possible to do everything on the Mac via the keyboard. You'd have to be physically handicapped in order to want to do so. Windows and X-Windows have more usable keyboard interfaces. NADBBA.
• Time Machine is very nice. Plug in a USB drive, answer the prompt with 'Yes,' and you're backed-up. Restores are 'intuitively obvious.' Yes, I know all about rsync, unison, tar, WinZip, etc. Time Machine is a premier example of Apple's obsession with UI.
• It is stable. Very, in my experience. From what I've read, it is even stable on non-Apple hardware ("Hackintosh").
• Spotlight is nice. Almost as nice as Time Machine.
• Quicksilver is astonishing.
• There are lots of nice, elegant programs that are Mac-only. Quicksilver, Circus Ponies Notebook, some spiffy text editors, etc.
• There is lots of hardware that doesn't have a Mac driver. You can say this is not Mac's fault. I agree. It still doesn't solve my problem of not being able to use my Canon MF 5770 printer with OS X. (Yes, I did set up a PostScript virtual printer in a Windows VM and print from the Mac via it. If I had to, I could do so on an ongoing basis.) NADBBA.
• The Mac version of Quicken is the seriously retarded sibling of Quicken for Windows. Data import from QW to QM is highly lossy for investment accounts. You can say this is not Mac's fault. I agree. It still doesn't solve my problem of not being able to use Quicken on the Mac. (Yes, I can run QW in a Windows VM, and even do so without the Windows desktop appearing on the screen.) NADBBA.
• Double-clicking, shift-clicking, Alt-clicking, Shift-clover-alt-control-twirl-clicking. NADBBA.

I could definitely use Mac OS X if I found myself in an environment where it was provided. It has its issues, but most of them I can work around or live with.

If Mac became my primary platform, I'd become one of those shrill voices always complaining about vendors who fail to provide OS X drivers, and provide sub-par Mac versions of their software. It didn't take long for me to become seriously ticked-off at Canon for not providing access to my borrowed Mac. I was furious that Intuit marketd its Mac product as "Quicken," when it was less capable of importing Windows' Quicken data than Gnucash2.

I don't want to go around with a chip on my shoulder, no matter how 'justified.' Vendors treat Windows as first class out of self-interest. I don't blame them.

My 'standard' set up is Ubuntu as the root-level OS (the real machine's OS), with Windows 2000 in a VM as my Windows PC. I really do all my computing in Win2K, treating Ubuntu almost strictly as a hypervisor. I run Win2K because I can take my USB drive with Win2K and plug it into almost any PC, and run *my* copy of Win2K without any product activation woes.