The current Xfce umbrella package in both Debian stable and Ubuntu 9.10 is xfce4 (version 4.4 in Debian, and 4.6 in Ubuntu). After you've installed it, log out of X. If you're running gdm or a similar app as your login manager, check the bottom left of the screen for a "Sessions" option, and you can choose Xfce for your next session. When starting the session, you'll then be asked if you want to make this your default window manager.
The first thing I noticed was how fast it started up compared to Gnome. Admittedly my desktop is reasonably old, so the difference might show up less on a newer, faster machine, but it was a very pleasant surprise to be started up so quickly. I also tried it out over VNC on my local network, and running inside a virtual machine, and for speed alone, I'd rate it significantly better than Gnome or KDE. Once running, the speed and usability increase continued; things like the system menus and settings were noticably faster to come up than on Gnome or KDE.
There's a "Tips and Tricks" box displayed by default on startup (this can of course be switched off), from which I discovered that you can get a xfrun4 application launcher dialog box by hitting Alt+F2. This works as advertised, and is a nice shortcut for those who prefer the keyboard to the mouse. Other desktops do also have similar launchers available (e.g. gnome-do, which has other useful functions as well), but the Xfce one is simple and fast.
The update manager fires itself up automatically and seems to work well; you can look at the recommended updates in whatever detail you choose, and install with a single click. You can also turn it off via the Settings-Autostarted Applications menu item. If you want a seriously stripped-down desktop, you might feel that the default set of autostarted applications is too many; but for the majority of people, having the update notifier, print manager, network manager, and so on autostarted will be welcome.
Read the rest at Linux Planet.