When users have Office Communicator 2007 R2, they will have access to all new features of Office Communications Server 2007 R2 that you have enabled. In addition, there are changes to the user interface of Communicator 2007 R2.
As you install and enable new server roles in your Office Communications Server 2007 R2 deployment, the features that those server roles provide will be available to users of Communicator 2007 R2.