I think it looks pretty damn hot
The M3 is becomming a bit more smooth, stylish. This and the RS4 have the "chunky", steriod beefy look I liked in the E46 M3.
M3 = more sport, track duty, engaged spirited driving
C63 = if I have to drive around clients + kids, lots of highway driving, nice M5 alternative
Both serve their purpose, both fantastic choices. What a great time to be an enthusiast, eh?