Well we've got our CRM 2006s/2 (AKA CRM 5.2) up and running and the most immediate change is the new UI. 

CRM 2006s/2 has a bit of a mixed history and is principally positioned for customers new to CRM who wanted an easy way to transition to CRM 2007.  Originally CRM 2006s/2 was never going to see General Availability but last month SAP changed their mind and announced that it was going to be generally released from December 2007 and would be in support until 2010.