I installed Zimbra on Ubuntu 7.10, thanks to the wealth of information posted on your forums.
We are currently migrating from Exchange server 2003, I tried using Zimbra's migration tool , then later changed my mind. Instead, I used IMAP to pull all mails from the Exchange server to an additional account created on the ZImbra server. We still have Exc 2003 running, so that makes the transition very smooth and user friendly.
Unlike Exc 2003 POP3 connector that pulls mails every 15 minutes, Zimbra( Fetchmail) pulls your mail anytime you click on getmail. I think that is real cool. We've had to manually pull mails for our users, especially when they are in a hurry to read mails.With Zimbra, that nightmare is gone for good.
The IM works too, I got the info from the forums too, we are currently using Pidgin.
Using the IMAP, Zimbra was able to pull all our calendar entries on Exc 2003.
Overall, I think Zimbra rocks!!!!!!
I am already spreading the good news, and will get it installed in as many organizations as possible.