I got my first Merc today, a C350 SC. It has the HK upgrade and it sounds pretty good, I usually fit aftermarket poweramps and speakers to my cars but this one is good enough. I didn't think the standard Audio 20 setup was much good at all.
The HK setup in some of the big Chryslers is excellent. The Dynaudio setup in VW Passats and Bose setups in Audis I have heard are all pretty poor. I have heard some really good things about the Martin Logan configs in some Lexus models.