XFCE was originally a visual clone of CDE, but coded in slightly less obtuse widgets (GTK vs Motif). XFCE has remained pretty great, supporting the new, but still keeping the spirit and speed of the old.

I used CDE on Linux for few weeks on laptop (IIRC only weirdness involved in building it was Imake, which makes sense for pre-X.org X11 related stuff).

It works perfectly for the same values of "works" and "perfectly" as on commercial unices. In other words it is in the middle between lightweight WM and full desktop environment, mainly because there aren't any applications that meaningfully integrate with CDE apart from all the dt* stuff (text editor, terminal, calculator...) included with CDE distribution and for the CDE's design to be meaningful you really want CDE applications that integrate with it's object model and not just plain X applications, otherwise it is only somewhat mis-designed window manager.

As a WM it is VERY useful and wraps windows for modern apps without issue. Some of the CDE apps though (dtcalendar and dtmail) are in need of a complete overhaul to be useful with modern protocols.