It's not just Xubuntu, but other distros such as Salix OS and VectorLinux are benefiting from XFCE improvements. I switched all our systems over to Xubuntu just before 10.04 ran out and have been testing other distros that offer the XFCE DE. So far, every distro I've tried with XFCE has worked OOTB.

It's rock stale and quick on my I5 desktop. Even our laptops have seen performance improvements. Plus, those who enjoy the benefit of configuring their DE will find it quick and simple. I totally agree with VTPoet:

It's going to be interesting. XFCE has a reputation for being lightweight. With more users and success, there's going to be a push for more features.
Just my $0.02