After much research, I too am going for the rear-mount rack. Thule Hitchpost most likely.
While a hitch may look out of place on an E420, a simple black plug will disguise it well enough I reckon.
Also -
1.) Greatly reduced windnoise.
2.) No deforming of rubber door seals.
3.) NO contact with paint anywhere (rack or bikes/skiis)
4.) No chance of bike or skiis slipping and dropping onto painted surface -
ouch! 
.
5.) No extra effort to lift a bike overhead after a hard day's ride (which increase the chances of #5 occuring
)
6.) Takes about 2 seconds to remove when not in use.
Sure, they don't looke as 'sporty' as a roof rack, but for functionality, there's no comparison. And my car will thank me in the long run.
Regards,
- Ryan