Many major car magazines where invited to England in winter of 2004 by GM to test drive an engineering "mule" (built for testing). Results from these early tests were published in the spring in most major car magazines. Check this forum's news section for discussion and links to those articles.
Adding links to most of test mule articles from May/June 2004:
MotorTrend
AutoWeek
CarDriver
Automobile Mag
Also adding PDF files to save articles.