I installed xubuntu-desktop (sudo aptitude install xubuntu-desktop) in Linux
Mint (which natively has Gnome desktop) and I used the xfce session a few times,
I also installed updates and then it happened twice that the panels disappeared
after logging in, but this was solved after two restarts. Then I got back to the
Gnome desktop environment (Gnome session) and it seems that it is mixed up with
some elements from xfce. The panels disappeared once again but after two reboots
(sudo reboot) they are back. Now some icons on the panels are different and
Firefox opens a different site by default, that of Ubuntu 8.04, not the Linux
Mint site.
It is interesting and it was a little bit scary and a minor annoyance, not a
real problem.
By the way, I find xfce quite fast and quite practical, except that it does
not see the partitions of other Linuxes on the hard drive. I also have Kubuntu
installed.