As if the R-Class' styling didn't make it unique enough, it's also one of the few models in the U.S. with an available clean-diesel engine. The diesel V-6 is rated 25 percent better in EPA-estimated combined fuel economy than the gas V-6 engine, but as diesels are prone to do, it also makes more noise. All versions of the R-Class have all-wheel drive.

I've been impressed with the immediate power delivery in many modern diesels, but you don't get that feeling in the R350 Bluetec. The specs say the torque is there — 400 pounds-feet at a low 1,600 rpm — but it seems to arrive late. It's not a problem on the highway, though, as the diesel has plenty of reserve power for passing.

The R-Class' 210-horsepower, 3.0-liter diesel V-6 is clean enough to be sold in all 50 states. Mercedes' Bluetec technology reduces diesel emissions by squirting a solution — AdBlue urea — into the exhaust gases. This process creates ammonia, which in turn changes up to 80 percent of nitrogen oxides (a pollutant that contributes to smog) into nitrogen and water, according to Mercedes. You'll consume less fuel with the diesel V-6, too. It gets an EPA-estimated 18/24 mpg city/highway, whereas the gas-powered R-Class gets 15/19 mpg on required premium gas. The same diesel V-6 engine is also offered in the M-Class, where it's rated 18/25 mpg, and the GL-Class, which returns 17/21 mpg.

The diesel R-Class costs $1,500 more than the base gas version, but the diesel's greater efficiency should make up the price difference in a little more than two years, based on current fuel prices and assuming it's driven 15,000 miles a year.