Am on my 2nd E320 CDi and found them almost totally reliable.
Only fault in a total of about 60k Miles on the 2 cars were:-
Passenger seat detector for child seat went U/S.
Diesel engine is a lot cheaper to service but on advice I do change Oil & Filter at about 8k miles between indicated services.
Finest car's I have owned in 40+ years of motoring.
I get 40 MPG (UK Gallon) on run and 30 MPG Day to Day.
Performance is better than the E320 Petrol due to Torque being more than a V8 5.0.
The biggest problem is driving it within the speed limits.
We will have a V8 4.2 available to us next year. That will make it to US soon as well.
Do a SEARCH on my name (Mike Stone) and you will find a number of posts with more info.