We’ve done a brief side-by-side comparison of Microsoft Exchange vs. Zimbra features. For those of you who don’t click on links, here’s the comparison:
Cost
Zimbra hosting is more cost efficient than Exchange hosting:

Zimbra hosting: $3.95 to $9.95/mo per user
Exchange hosting: $9.95 to $24.95/mo per user

Flexibility
Use Zimbra on any platform (Windows, Apple, Linux), with any mobile device [...]