No Operating system can beat Mac Os X Leopard User Interface, even not Windows Vista and Windows 7. Although in terms of Simplicity of usage Windows Operating system are more preferred then Mac OS X.
If you like to have Mac OS X Leopard like look and feel on your Windows Vista then here Windowsblind Mac Os X Leopard Theme for Vista which will transform your Vista to Mac Os X. In order to use this theme you need to have Windowsblind installed on your Vista.
Screenshot of Mac OS X Leopard Theme for Vista