After the window manager and other display options, the most important aspect of a desktop environment is the utilities designed to run with it.
In this area, Xfce cannot compete with either KDE nor GNOME, both of which have dozens of specialized utilities. In particular, Xfce could do with small utilities such as a calculator, and options for configuring a printer and other hardware or installing fonts. However, given the wealth of existing choices, especially generic ones like Firefox or OpenOffice.org, this lack of choice is not crucial in a modern desktop. It doesn't hurt, either, that Xfce opens KDE and GNOME applications faster than either opens each other's.
One utility that Xfce does include is Orage, a calendaring system that closely resembles GNOME's panel calendar display. You set appointments in Orage by selecting a date then clicking File > New to fill in the details and double-click a date to see a list of the day's appointment. Past appointments are archived, and you can set the size of the archive to avoid it taking up too much hard drive space. Like the rest of Xfce, Orage is simply but adequately featured, and should be usable by anyone familiar with desktop environments without any problems.
Another major utility is Thunar, Xfce's file manager. Although similar to GNOME's Nautilus in general appearance, Thunar opens in about half the time, and jettisons many of Nautilus' features, such as emblems for classifying files and the ability to burn CDs from the window.
Nor does Thunar have Nautilus' option for a Delete menu item that bypasses the Trash; you simply have to know to press the Shift key when selecting Delete -- a feature that is either a welcome failsafe against carless mistakes or a nuisance, depending on your habits. But in compensation, Thunar boasts the ability to associate a custom action for one type of file, and an option to choose whether actions on a folder, such as a change in permissions, affect its contents.
Choosing to Use Xfce
Whether you use Xfce depends on your preferred tools and workflow. If you depend, for example, on having a character map utility for international letters on the desktop, then Xfce may not be for you. At the very least, you will need to be prepared to search for a replacement utility, or check how a GNOME or KDE applet works on Xfce.
However, once you have the desktop configured to your liking, you may find that Xfce provides the best of both worlds, combining the speed of minimalist desktops with customization and a balanced feature that approaches those of heavyweight desktops like KDE and GNOME.