I just upgraded to the newest mac os, Leopard, yesterday. So far so good. The only new features I have used are stacks and quicklook. These are both very cool features and work as advertised. I have not noticed any glaring bugs yet, but all of my printers were deleted after the upgrade. Also, Yahoo chat no longer worked, but luckily Yahoo just released a Leopard compatible upgrade with lots of new features. This brings more parity between the popular chat client on both the Windows and Mac platforms. It also seems to play even nicer with a Windows network. I have not done much digging with this, but it seems like they are getting closer to Windows-like single sign on when you have the AD plugin installed.