Apple shipped the OS X update Leopard last week. The upgrade of the Macbook in our office was extremely simple and the new features and UI improvements are worth the money for Leopard.

Engadget published a feature by feature comparison of Apple's Leopard versus Microsoft Windows Vista. Leopard scored a bit better in that comparison.

I consider myself an early adopter but I still havn't upgraded my Windows PCs to Vista. It is just too big of a step and there are still too many questions for me on potential hardware, driver, and software issues that the benefit of a new OS is just not there right now.
With Leopard, you just pop in the disc and you do not have to think about hardware or driver issues, if your Mac is not too old. There are no negatives and plenty of positives which makes upgrading a pretty easy decision to make.