You can use the Openbox manager definitely, where the line blurs a little is that Compiz/Beryl need to act partially as a window manager to provide compositing. This is the case also with KDE and Gnome. Essentially your window borders and virtual desktops are drawn by compiz. What this meant in KDE is that the switcher applet didn't work, but you used the key combos or mouse to switch.

I'd be quite interested in seeing the other WMs being used, everyone seems to run KDe and Gnome. Suddenly I feel the need to run it under TWM...