The users have only one complaint: they cannot set up an Out of Office auto-responder like they could on Exchange. I thought that was good, and tried to explain why auto-responders range between useless and evil, but had no success. They want it anyway. So I'm setting up vacation in their .forward files when needed, and looking for a good web interface so they can do it themselves. The Webmin interface I tried didn't work well, so I'm still looking, and may have to work on the Webmin module myself.
I came across the same problem. Looked into using the horde module, but never got it working right (since I wasn't using horde in the first place, etc.). I ended up using it's idea of operation and threw together a php script that will do it. Needs ftp on the server to work. If you're interested, let me know.