I've been using the server for about 24 hours now (well, truly it's been about a week in a VM but I'm not sure that counts).

I've noticed a few usability items you guys may or may not have known about... this is all on FireFox 1.0.7 on MacOS talking to the beta2 open source:
* There are a lot of bugs in the filter UI, but if you massage it just right, it will work (Kevin says this is known)
* Expanded folders are not saved; if I expand my Inbox to show various folders and then go to the calendar and back, I have to re-expand. Ditto between sessions... if I log out and log back in, they're collapsed.
* Ticked/checked calendars are not saved; I've subscribed to the "US Holidays" iCal URL. If I check this calendar so I can view it, then go to the inbox and return to calendar, it's unchecked. Ditto with sessions as above.
* Administrative console directs me to the wrong URL when I want to view another user's mailbox. I have my server setup for https only and the admin console redirects me to http.

-Chad