Cool functionality means..
Search, Shared Calendar, Collaboration, GAL all other things why Zimbra exist at first place not just to read mail on web.
Yes if thye give you all the API you need you will be spending LOT of time trying to make your own interface ..which is kind of lot of work
Rest for academic purposes its good to lean all API stuff and see how you can hook up your existing system..but again then you not using Zimbra for what its made for.
Raj S Vrach
i2k2systems.com