On a related note (i.e., non-diesel), I just went on a 1200km (750mi) trip in my 5-speed manual 190e2.6, and my overdrive 5th gear definitely helped in the fuel economy area, but I had to downshift into 4th sometimes to make any progress on inclines. This was because I had to drive within the tolerated speed limits (around 120kph/75mph). If I drove above 140kph (88mph), the engine would have been in a fatter part of the torque curve, and I would not have much need to downshift.

I came out really amazed however, by the seats, having had no back strain whatsoever after that long drive. They look so innocuous, and only reveal themselves after a long drive.

Another thing I appreciated was the effectiveness of my Euro-headlamps.

Other small details I appreciated were the dead pedal and the wide steering wheel, which lessens the strain on the arms on a long trip.