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.
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. This also makes it almost impossible to use Xfce >=4.15 components on traditional Window Managers like: Openbox, Fluxbox or IceWM.
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.
Recommendations
Comments
Out of Date Pros + Cons
Con No HiDPI support
Since Xfce is still based on GTK2 there is no HiDPI support (scaling UI elements).