Xfce has been my darling for quite some time now. Along with being extremely customizable, it can be quite light on resources and hence efficient on older hardware. Unfortunately, most of the users seem to read this as Xfce is good only for the older machines. Not so!
Xfce is one of the best desktop environments even for the most modern machines. Like any other desktop environment it can sit pretty with its inbuilt composite window manager, flexible theme support, intelligent panel plugins, smart session management and adhereance to free desktop standards. Where Xfce really shines is that it supports some unique features like desktop menu, minimizing applications to desktop and easily configurable keyboard shortcuts.
If you are reading about Xfce being suitable to older hardware, it has a lot more to it than just that. You can mould it as you want, and it won’t come in your way.