I'm sure Helix GNOME is old news by now, but recent news about GNOME Foundation prompted me to install GNOME (I prefer plain WindowMaker).
I just installed Helix GNOME. Very slick and easiest GNOME installation I've ever experienced.
Of course I had to immediatedly try some coding too... see this applet. Coding that little applet was pure joy. I've done some plain gtk+ coding before, but this was actually my first attempt at GNOME spesific code.
Helix GNOME has very polished look with good programming APIs and infrastructure (CORBA, bonobo etc.). No wonder that Sun and other companies chose GNOME to replace CDE.