As per the initial development contract the SOGo webmail client was developed to look like the Mozilla Thunderbird client. The SOGo calendar initially used the user interface of the regular OpenGroupware.org server.
This version of the interface is still available in the 'skyrix' and 'agenor' branches of SOGo. It uses a mild level of AJAX (eg to toggle mail status fields or to add recipients without reloading the HTML page).

In 2006 Inverse started to rework the web interface. The 'inverse' branch contains an interface which makes heavy use of AJAX. Further the scheduler was reworked to match the design of the Sunbird application.
You can take a look at the interface on the screenshot section.

The SOGo web interface can be localized in an easy way. The layout of the application is template based and can be adopted to site specific requirements.