A simplistic view.. if yours users want Outlook, then let them in Outlook.Create a webapp skin that mirrors outlook.
As a fan about Zimbra ZCS have to admit that ZD it's not yet ready for mainstream.
So, keep your fat clients, whatever you want.
Speaking myself, think Thunderbird + Ligthing + Zindus can do the job. Obviusly, it's not an Outlook replacement.. yet. But it's free and FOSS.