Recs.
Updated
Xfce is a low resource user friendly desktop environment created for UNIX-like operating systems.
SpecsUpdate
Pros
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 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.
Cons
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.
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 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 Officially is no Longer the Champion of Lightweight Memory Usage
According to multiple credible sources in the Linux world and the KDE developers, XFCE now uses more memory than Plasma 5.17, due to Plasma bringing better and faster updates. Even the XFCE folks admit XFCE is somewhat getting out of hand.
I suppose this was tested on a minimal install of Plasma, without the entire KDE ecosystem. So if you are concerned with memory usage and use a "build-it-yourself" distribution like Arch, avoid XFCE and install Plasma by itself.
Con Will become more interconnected an less modular
Isn't as modular anymore as it was 10 years ago. In the future (4.16) they will also introduce GTK Client Side Decorations to all setting dialogs so you will have some problems with using a different Window manager or when using xfce components outside of Xfce/GNOME.
Con One pixel wide window borders
The non-configurable, one pixel wide window borders make resizing difficult. Work-arounds exist but those are clunky at best.