At least XFCE 4.8 is hardly usable for the average user I think:
* The user switching applet only shows user names, which they don't know (seems fixed in 4.10, though)
* GTK+3 applications are unstyled by default. Given that you most likely need to use GNOME components to make the desktop somewhat usable, you need new themes.
* It does not feel "integrated" in any way.
GNOME 3 Fallback mode provides a more coherent and integrated environment than you could ever create with XFCE (apart from Totem and Eye Of Gnome, which look out place in Fallback mode, as they use the dark variant of the Theme, while Metacity uses non-dark borders).
Maybe I might have the chance to try XFCE 4.10 within the next year, and see what changed, but I don't think it can get much better than GNOME 3 Fallback mode for (my) average users.