I've been having a play with Xfce on one of my Solaris workstations.
And I have to say I've been very impressed. It's reasonably fast and light. It has an increasing number of applications. The file manager - Thunar - is quite nice.
There are two things that I think are in favour of Xfce. The first is that because of its common roots with gnome it can leverage all the gnome applications and GTK themes. The second is that it is very polished. There is a good section of window manager themes, and the artwork is very well done.
All in all it's well worth a look.