I've been playing with it occasionally at work over the last few weeks, but I've never really used Exchange so can't compare. Straightforward to install and set up at least, but I didn't really get anywhere when looking at the source. As far as I can tell, the offline usage is provided by local storage of your IMAP folders, or using POP as normal in the first place. Just got a network edition trial which has the outlook connector for syncing everything, but no-one here has used Outlook/Exchange much before, and as a small company we don't bother much with shared calendars or anything like that. The web interface is very slow on anything with <1Gb of RAM and it has slight but noticeable and annoying lag on my Athlon XP 2500, 1Gb workstation. Wasn't much slower over the wrong end of a DSL line than on the local network, so I can only assume the AJAX loaded web frontend is resource hungry.
I think it probably already is a viable alternative, but the problem for Zimbra is that Outlook is so entrenched in business that any expansion of email services and features will always favour going down the Exchange route.