No one buys a Mercedes-Benz S-Class by accident. This is Mercedes' largest and most comfortable sedan. By no coincidence, it is also the most expensive. An S-Class is a rolling, four-door announcement. It says you know what you're doing with your life, and while you have money to spend, you do not spend it foolishly.

Or at least that's the stereotype. In reality, gobs of people buy S-Classes without thinking about it. Parking lots from South Beach to Palm Springs are dotted with the cars, and New York City boasts almost as many chauffeured S-Classes as it does yellow Ford sedans. For many people, an S – the letter stands for the German word sonderklasse, or "special class" – is simply a status symbol. It's the cushiest, fanciest sedan from the oldest and most respected carmaker extant. What more could there be to consider?

Lots, actually. Mercedes-Benz has been making cars in one form or another for 125 years. You do not stay in business that long, no matter the product, without being good at what you do. Similarly, while there are a lot of S-Classes on offer – in the United States, the model range encompasses six cars and runs from the $92,275 S400 hybrid to the $211,775, 621-hp S65 AMG – each is a distinctly different machine and does different things well.

Still, most S-Class buyers seem to just waltz into a dealer and pick up the most expensive version of the car they can afford. At the risk of being blunt, this is stupid. And the 2012 S350 Bluetec 4Matic diesel, the latest entry into the S-Class range, is proof.

Let's take a brief detour and examine the rest of the S-Class range. At the bottom, there's the aforementioned S400 hybrid. It gets 19/25 mpg city/highway and is the slowest S to 60 mph. Directly above that is the S350, which we'll get to in a moment. One step above lies the $95,375 S550, which offers a 429-hp, 4.6-liter, twin-turbo gas V-8 and 15/25 mpg. Above that is the 536-hp S63 AMG (twin-turbo V-8, 15/23 mpg, $140,175), the 510-hp S600 (V-12, 12/19 mpg, $160,375), and the twin-turbo, V-12-powered S65 AMG (12/19 mpg).

Confused yet? Each of these cars is suited to a specific customer. The S400, for example, is best for captains of industry with a long, chauffeured commute in stop-and-go traffic; its forte is keeping gridlock green while still making you feel like you've arrived. The S550 is what you buy if you want a good all-rounder, neither outrageously fast or wanting for power. The S63 is the canyon-carver special, loud and fast enough to satisfy your hot-rod urges but lacking the nose weight of the V-12 models and thus relatively light on its feet. The S65 offers autobahn-crushing power and remarkable refinement with a bit of the S63's edge and exhaust note. The S600 pairs most of the S65's grunt with the understated grace of the S550. It's best for those who want to own the highway but don't want to be obvious about it.

Where does that leave the $93,425, diesel-powered S350? Easy: It's what you buy if you're smart.

The S350 is the first diesel S offered in America since 1995. It's also the only full-size diesel luxury sedan you can buy in the U.S., Mercedes or not. Its turbocharged 3.0-liter V-6 produces 240 hp and a thumping 455 pound-feet of torque at just 1,600 rpm. (This sort of power delivery is common with diesels, which generally thrive at low rpm.) 60 mph comes up in 7.0 seconds. Fuel economy is 21/31 mpg, an astounding figure given that the S350 weighs almost 5,000 pounds. A version of Mercedes's excellent seven-speed transmission is the only gearbox available, and all-wheel drive is standard. (The latter is optional on the S550 but unavailable on other S-Classes.) The end result is one of the best big sedans on the market. In the interest of science, I drove an S350 from Chicago to Key West and back in the middle of winter. The idea was to capitalize on the S350's strengths – tremendous range, decent fuel economy, peerless comfort – while figuring out just who, exactly, would want an oil-burning autobahn sled.

Like all S-Classes, the S350 is basically a rolling living room on wheels. You can buy it with massaging front seats, each of which comes with multiple massage styles (From the menu, and I quote: "Fast and vigorous." "Slow and gentle."). Headrest video monitors are an option, as are electrically reclining rear seats, a split-view front screen – the driver can view the navigation system while the front-seat passenger watches a DVD – radar cruise control, and a host of other high-tech options. In typical Mercedes-Benz fashion, all of them work exceedingly well and do exactly what they're designed to do, which is more than you can say for most of what's on a modern Lexus.

But the S-Class's interior appointments and available tech aren't the main draw, nice as they are. The big pull is that the S350 is a no-excuses large diesel sedan, with everything that entails. To wit:

Economy: Like a lot of modern diesels, the S350 delivers more than what its EPA numbers might indicate. Over 3,000-plus miles, I saw an average of 25 mpg in city use, 32 mpg on the highway. Range varies with use, but figure on between 500 and 700 miles a tank, depending on use. This is outstanding. While rocketing down I-65, my bladder was repeatedly outlasted by the car's fuel tank.

Torque: You get lots of it, and everywhere on the tach. The seven-speed automatic is a bit slow to downshift but goes about its business smoothly, launching the car away from stoplights with a velvety, refined surge. The only thing really missing is an excess of highway passing power – traffic maneuvering at speed sometimes takes a bit of planning.

Noise: You get little of it. Diesel clatter is audible from the street but almost never from the cockpit; At 90 mph, there's a slight wind whoosh over the A-pillars, just enough to remind you that you're moving.

Highway stability and long-distance comfort: Oh lord. Oh lord, lord, lord. If there's a greater highway joy than cranking out thousands of miles behind the wheel of a large, pavement-crushing Mercedes, I don't want to know about it. As a driver, you're given just enough information – just enough steering feel, wind noise, and body roll – to stay awake, informed, and interested. As a passenger, you tend to fall asleep a lot, which is the nicest compliment anyone can give a big sedan, no matter the price.

Either way, you climb out of the S350 at the end of a 12- or 14-hour drive feeling refreshed. When I hit Key West, it was like I'd spent the past three days lounging on the couch, not hammering down the highway. At 70 mph, the engine is spinning at just 1,600 rpm in seventh gear. The steering is heavily weighted and slightly wooly but incredibly pleasant; it feels like you're commanding a helm change from somewhere in the car's bilges. (This isn't a knock. The steering fits the S350's torque delivery and laid-back personality.)

The point here is balance. The S350 is a subtle piece, an advanced, confident car that doesn't broadcast its ability or price tag. It offers all the power you need, remarkable fuel economy, and just as much comfort as anything else on wheels. In that sense, it's the most old-school machine Mercedes offers. It harks back to a time when luxury cars were sold on how they made you feel, not how they made you look. Because it trades flash for capability and excess for practicality, it oozes intelligence like few other modern machines.

The irony is, people don't buy luxury cars with their brains. They buy them with their hearts, which means that the S350, like other diesel S-classes before it, will probably never outsell its gas-powered siblings. What a shame.

WIRED Impossibly refined. Sips fuel. Makes 100 mph seem like walking. Compresses time and distance like nothing else in the business.

TIRED Wants – ever so slightly – for highway passing power. Diesel V-6 is quiet from the driver's seat but clattery from the curb. Touchy brake pedal requires a frustrating amount of attention for smooth operation.

All photos: Sam Smith