Aluminum vs. Composites: Five Myths About Composite Aircraft
Myth 1 - Composite aircraft can be built with fewer man-hours than aluminum aircraft
Myth 2 - Composites are lighter than aluminum
Myth 3 - Anytime you put compound contours in sheet metal, the costs go out of sight
Myth 4 - Composite materials cost less than aluminum
Myth 5 - Smooth composite airplanes provide laminar flow and low drag

Why Are Composite Aircraft So Popular?
In the case of Cirrus, because they look good! They are well finished and the company has incorporated modern avionics. Nobody is claiming they are the world's most efficient, because they do not beat a 40 year old Cessna.

Other Reading
Stephan Wilkinson - The Fundamental Truth
Beechcraft's Composite Challenge
A Cessna Foray Into Composites
Adam Aircraft
Burt Rutan - Pied Piper of Composite Aircraft
Paul Whetstone's views on Composites vs. Aluminum
Japan's Composite Wing F-16 Failure
Winning The Battles - Losing The War