I got my first MB in June 2004 when I got a brand new C270CDi Avantgarde. I have now done 50,000 miles without a single problem until last week (a week before it goes back to the contract hire company!!) The car is running very badly and belching out black smoke!! MB Dealer thinks it is just a quick re-programming job on the engine management side of things. The problem is I had it Superchipped shortly after I got it, so I now have Superchips out tomorrow to return it to default settings before taking it to MB to be checked. Superchips said they were pretty sure that MB would not detect it had been done, "pretty sure" was not good enough for me so I am having it returned to normal before going back to the garage with it.
Apart from the problem at present, the car has been faultless in every respect. It has had in excess of £8,000 spent on accident damage repairs but that was my own doing and no fault of the car.
The superchipping made a big difference on performance and fuel economy. I drive my cars VERY hard (4 sets of tyres in 50k miles!) and still average 38-40 mpg. The superchipping did make it smoke a little bit more than usual but apparently that is quite normal on the 270 engine.
I took delivery of my new E320 today and love it, the performance is great and the car is very quiet and smooth. On it's first run today I averaged 34mpg but I am sure once it has done a few miles this will improve. I will still have this one chipped just for the increase in fuel economy.