Leopard
This has pretty much been blogged to death throughout the interwebs so I won't bother adding much more noise on the subject except to say that I continue to be impressed by Apple software.  I upgraded to Leopard today, and as usual, it was a painless process with significant improvements (whereas Tiger was underwhelming, Leopard is a quantum leap).  Time Machine is probably the most valuable user-facing feature in the whole package--you just plug in a drive and back up, so when (not if) your primary drive crashes, you no longer have any excuses. 

Here is the only Leopard review you'll ever need.

My big beef so far:  what is the fascination with translucent windows/toolbars?  A little bit of that can go a long way, and Leopard takes it way too far in the Vista/Aero direction.

I don't use the Dock much, thanks to Quicksilver, so I don't have nearly as much of an issue with the UI changes there.

Otherwise, Leopard is pretty awesome.