I wouldn't go that far. Zimbra was the first client I was willing to use over Mutt. It taught me the value of Gmail-style conversation threading. It has strong IMAP support, and a reasonable J2ME client. It never ate my data, and I was able to use it on a daily basis for a year.

"Gmail killer" is wonderfully hyperbolic, but in its space, it's really not a bad option. Especially when measured against other self-hosted webmail + calendaring solutions. Of which there are two: Horde and Outlook Web Access.

I'm personally pulling for Bongo (http://bongo-project.org/), but I think it's more likely that this space will be ceded to Google Apps.