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Xfce is a lightweight desktop environment for UNIX-like operating systems. It aims to be fast and low on system resources, while still being visually appealing and user friendly.
SpecsUpdate
Pros
Pro Classic and familiar
Xfce works very much like the classic Gnome & Windows desktops, taskbars (panels) and desktop icons, letting you get your work done without being frustrated.
Xfce embodies the traditional UNIX philosophy of modularity and re-usability. It consists of a number of components that provide the full functionality one can expect of a modern desktop environment. They are packaged separately and you can pick among the available packages to create the optimal personal working environment.
Pro Adheres to standards
A priority of Xfce is adherence to standards, specifically those defined at freedesktop.org allowing for interoperability and shared technology for X Window System desktops. This interoperability is particularly significant for users looking to, e.g., run alternative window managers.
Pro A true UNIX Desktop Environment
Xfce adheres to the UNIX philosophy, which means it strives for being modular, minimal and expandable. This makes it very much customizable. You can make it as minimal as you want and as heavyweight as you want depending on the features and modules/plugins you use.
Pro Window manager (XFWM) is a compositing WM by default
By having a compositing WM as the default WM makes way for a lot of visual tweaks and tricks that can and do make Xfce look great. You can adjust the transparency, shadows, borders, etc. and many other advanced tweaks are also available.
Cons
Con Missing some basic functionality for a desktop environment
Xfce is missing essential functionality like a file-archiver or a polkit-client, so you have to find alternatives for those applications (eg: by stealing them from MATE or GNOME, however this adds additional dependencies that will bloat Xfce).
Con Now with Client Side decorations
Recent development versions introduce GNOME-ClientSideDecorations for some Xfce applications. Like on GNOME this breaks the overall consistency of the desktop. Eg: GNOME and some Xfce applicaions will use GNOME based interfaces like CSD's and popovers while the most other will use normal titlebars and popupmenus.
Con Screen tearing issues
The built-in compositor for Xfce does not handle VSync, meaning that it does not address screen tearing for those with Intel integrated graphics. A third party solution will have to be used for those that do want VSync such as using Nvidia proprietary drivers to handle VSync or installing a third party compositor such as Compton.
Recommendations
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Flagged Pros + Cons
Pro Highly customizable
Xfce offers plenty of settings, and even things like theming XFWM is a simple task (it's just a handful of images.)
Many possible permutations of window colors, borders, fonts, etc. Compositing can make it look downright sexy.