I'd rather have a complete IMAP client!
I just loaded the new Zimbra Desktop 0.51 (I'm a new Zimbra user all the way around) and was pretty disappointed when I discovered all I was getting was a duplication of the same stuff on my browser but replicated to a local host. While I can see why remote users (laptops and bad connections) would want this, for my use on a good LAN it doesn't help at all.
In comparison to the current implementation of Thunderbird (22.214.171.124 with Lightning 0.5) feels sluggish--everything takes time to sync, actions take time to happen, etc. This is on an XP client with dual-core 3.0GHz, 1 GB RAM, 100MBit LAN to the server.
I (and my user base) would much rather see the resources that are being expended on this effort, go into making a tight integration with a good open-source IMAP client like Thunderbird plus Lightning, which our people would use. All we'd need for that integration to be complete (as folders and calendars are already working) would be a common address book and better integration of spam classification.
Hope this isn't too politically incorrect :D but that's how I see it. . .