Every one compares the cost of diesel to regular gas prices. The fact is, BMW, Mercedes, etc. all require premium fuel. In Canada, diesel costs about the same as mid grade gasoline, which is a bit above regular grade and a bit below premium. In some places it sells for the same as premium.

Even with diesel prices above premium, there is still an arguement to be made for diesel (as long as the diesel option isn't too expensive) similar to the arguement made for hybrids.

In any case, I'll lease my next vehicle with a fixed residual and take a wait and see approach. If diesel continues to ride faster than premium fuel, then when it is 30% higher in cost, operating costs (fuel) will be a wash with a gas powered vehicle. In the mean time there is a substantial savings.

In addition, I just feel better knowing that I am consuming lelss fuel than the same vehicle with a gas engine.

As to the option cost, this is really anyone's guess. In the Mercedes, the diesel option returns performance similar to the ML350 engine. In the X5, the diesel option actual provides an improvement in performance over the 3.0si. BMW may tack on an extra premium for the increased performance. Mercedes kept the premium at a minimum becuase you are trading fuel efficiency for reduced performance. With BMW, it looks like we would be getting a double benefit.