I normally use Evolution, have just started using Zimbra desktop and have found two show-stoppers. I am have also installed a test version of ZCS 5.0 to see if we can adopt Zimbra as our corporate mailserver.
Zimbra Desktop looks really good, and has the makings of great client, which can beat the socks off Outlook. It needs a few more smarts though.
1. Zimbra Desktop does not seem to have the ability to do a addressbook lookup against an external ldap server. Zimbra desktop needs the ability to lookup any ldap server for an address.
We use a simple ftp server to which each employee publishes freebusy information according to RFC 2445. http://www.ietf.org/rfc/rfc2445.txt.
Outlook and Evolution can lookup the ftp server and read freebusy information based on the email address of the meeting attendees. They will also publish FB information to the ftp site. While outlook is limited to FTP, Evolution will do it to webcal, webdav and ftp.