CCF 2008 is much more improved version over CCF 2005 and easy to migrate. Here is a brief list of new features that make it worthwhile switching from CCF 2005 to CCF 2008
New features:
CCF 2008 uses SharePoint Server 2007 portal for web portal access. Users can manage their accounts that was lacking in CCF2005.
CCF 2008 works now with Windows workflow Foundation, which is next Microsoft technology for defining, executing, and managing workflows.
Enteprise level Single Sign On. More powerful SSO architecture that allows you to manage different user id/passwords via one login data store.
Better intergration with telephony, emails, instant messaging and voice interactive systems.
Hosted Application Toolkit.