I'm confused...
 

what's the "C/S" button on the S550 (and some other cars?)

its like Comfort/Standard (i think) but what is it for?

Transmisstion Mode - Comfort (starts in 2nd gear and shifts at lower RPM) and Sport.

From the W221 Manual:

Program mode C:
Engine, automatic transmission and
suspension tuning are comfortable.
The vehicle starts out in second
gear (both forward and reverse) for
gentler starts. This does not apply if
full throttle is applied or gear
range 1 is selected.
Traction and driving stability are
improved on icy roads.
Upshifts occur earlier even when
you give more gas. The engine then
operates at lower rpms and the
wheels are less likely to spin.
Program mode S:
Engine, automatic transmission and
suspension tuning are more performance
oriented.
The vehicle starts out in first gear.
Upshifts occur later.

thanks, both of you