There's a great little display next to the i8 concept car in the Welt that helped me put the carbon roof into perspective.
3 cylinders with handles on top are filled with equal amounts of steel, aluminum and carbon fiber (respectively). Next to these are ruler-sized strips of steel, aluminum and carbon fiber. The strips are secured on one side and allowed to move freely on the other.
By picking up the cylinders of the different materials and bending the strips you get a great idea of their respective weights and stiffness. The steel was super heavy and super bendy, the aluminum was less so on both accounts and the carbon was light as a feather and ultra-stiff.
Not the most scientific comparison, of course, but it helps illustrate why a carbon fiber roof is a good idea.
I could take or leave the exposed carbon fiber...personally I wouldn't mind my carbon roof to be painted the same color as the body of my car. Regardless of how effective it is in the real world, I think it shows the attention to detail that went into the design of this car and I'd never sacrifice it for a moonroof.
/// E92 M3 Individual - Azurite/Rust ~ E34 M5 - Brilliant Red/Silver Gray