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.
One notable feature of Xfce's Settings Manager is that items are tightly organized. For example, instead of having separate listings for Keyboard layouts, Accessibility, and Keyboard shortcuts, the way that default GNOME does, Xfce places all of them under the general category of Keyboard, making all of them easier to find and quicker to customize.
The Xfce desktop limits panels to a utilitarian gray, but does allow you to change the width, height, and position of each panel. You can also autohide it or make it freely movable so that other windows are not obscured by it -- a feature that is most useful when a panel is set to Normal Width, so that it doesn't occupy the entire side of the desktop.
Like GNOME and KDE, Xfce includes a series of small utilities that you can add to a panel from a right-click menu. Many of these utilities are standard desktop elements, such as a notification tray, a window list, and a clock. However, Xfce also has a number of other utilities, such as an Icon box, which displays icons for all running applications, Mount Device, which tracks all filesystems on the computer, including external ones, Screenshot, and Keyboard Layout Switcher. You can also select Launcher to add an icon for another application by selecting Launcher, or SmartBookmark to add a launcher for a Web address. None of these are spectacularly unusual, but they can be useful, depending on your preferences and work habits.
For a long time, Xfce had no graphical means of adding icons to the desktop. Now, it does, but you'll have to search to find it. Right-click an existing icon, and select Desktop -> Create Launcher (or URL link or Folder, depending on your needs). The Create Launcher window opens for you to enter the icon text (Name), the command to start the application, and an icon from your system's library.