There's nothing new about a diesel-powered S-Class sedan. In the late 1970s, Mercedes-Benz added the 300 SD to its North American lineup and sold nearly 30,000 units. Though more fuel-efficient than the 450 SEL with which it shared a body, the 300 SD was noisy, slow and smokey.
Don't let that memory color your impression of the current S350 BlueTEC. Diesel technology has made dramatic advances -- the big sedan's 210-horsepower 3-liter turbodiesel engine is quiet, quick and clean. And it returns excellent fuel economy of 21 mpg city and 31 mpg highway.