I've been sitting on 9.04. It does the job. While 9.10 looked interesting it didn't sound that stable. Once 10.04's had a while to mature a bit I may do a clean install. After using xfce on one of our servers I think I'll go for Xubuntu 10.04.
We use ubuntu on our servers and its been great. The server install has some great options that allow you to tailor the install to the purpose of the server to avoid to much superfluous stuff from being installed. The xfce interface is also great on servers as you certainly don't need the Gnome eye candy when you're working on a server.
I usually perform CLI-only installs of the server edition, and the new versions (starting with 9.10) provide much better support for virtualized hardware from the get-go. Plus being able to install a barebones version for me is a plus, as all my VMs derive from one pre-configured master.
Tried 9.10 but it did seem a bit unstable. I am running it as my NAS at home, but I'm stuck with 8.04 for now at work).
"Roads? Where we're going, we don't need roads." - Doc Emmet Brown