Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Last year's DesktopLinux.com's survey showed Xfce was the third most popular desktop environment. Granted, it was a distant third to KDE's second place and GNOME's first place, but Xfce does seem to be gaining in popularity in the last few years.
Part of the reason may be the availability of Xubuntu, a version of Ubuntu that uses Xfce for the desktop. However, for the most part, Xfce placed strongly on its own merits, having largely outgrown its somewhat geeky origins in recent versions to provide a more lightweight desktop than GNOME or KDE, and enough customization to satisfy GNU/Linux users without overwhelming them with options.
Some of Xfce's programs, such as the Ristretto graphics viewer or the Orage calendar, are close equivalents of their counterparts in KDE and GNOME; they're functional, but not particularly different from what you've seen before. If you want to investigate Xfce, what you want to watch for are the features that are either unique or else essential or hard to find, like the ones listed below. They may just tip your decision about which desktop to use.
At the cost of a small bit of redundancy, Xfce lets you configure desktop elements either individually, or centrally from Settings ->Settings Manager. For example, if you want to configure the Thunar file manager, you can either select Edit -> Preferences from a Thunar window, or else Settings -> Settings Manager -> File Manager. If you're like me, you'll appreciate the Settings Manager when you are first setting up your desktop and use the preferences for individual applications as you fine-tune your desktop.