To the uninitiated, it looks pretty much like any other executive limo: handsome but not brash, confident but not arrogant, the sort of car the Belgian minister of finance would take to a meeting at the International Monetary Fund. But it’s a beast – a silent, merciless, all-wheel drive predator of overwhelming ferocity.

It is also a four-door sedan that will keep up with Lamborghinis, Ferraris and F-18s being flung off the decks of aircraft carriers by steam catapults.

Depending on your allegiances, the S8 is either the quickest large sedan in production or the second-quickest. It’s faster than an Aston Martin Rapide S. Faster than the Maserati Quattroporte. And faster than anything Mercedes currently sells under its AMG badge. The 520-horsepower S8 will blast from a dead stop to 60mph in an astonishing 3.5 seconds – neck and stylish neck with the 550hp Porsche Panamera Turbo S. The 2013 version of that Panamera, however, starts at $176,275 in the US, well before a customer hits Porsche’s very long options list. Meanwhile, the 2014 S8 starts at $113,395, netting the S8 buyer enough pocket change for a Porsche Cayman S. Porsche has announced that the 2014 Panamera Turbo S will see a bump in power to 570hp, and a corresponding bump in price, to $181,295 (or $201,495 for the more opulent Executive model).

Still, why is the S8, with 30 fewer horsepower than the current Turbo S,  such a stone-cold performer?

The answer lies in the S8’s compact, lightweight 4-litre, direct-injection, twin-turbocharged V8 engine – which by most measures is dramatically underrated. Dynamometer testing has indicated that the unit generates closer to 575hp. Maybe more.

But the S8 isn’t simply about straight-line speed. Built around an aluminium space frame, it is relatively lightweight for a vehicle of its size – never mind one that lugs around such a remarkable spread of technology. The frame also imparts exceptional structural stiffness. This isn’t a luxury car that isolates its driver from the road, but nor does it fill the cabin with raw ride motions and harsh noises. Instead, the S8 interprets its environment and reports its findings precisely to the driver. Paddle shifters respond unerringly quick, settling the car in whatever of its eight forward ratios that the driver selects. Should the S8 wander from its line, there’s a lane assist system to inform the driver that corrective action is needed. The effect is one of driving enhancement; the S8 does not presume to be smarter than its operator.

All flagship sedans brim over with technology, and the S8 has virtually every conceivable feature – including an onboard wi-fi network that lets passengers surf over a common internet connection. Electronics have become so ubiquitous that they arguably no longer distinguish one high-end luxury sedan from another. A good thing, then, that the S8 calls on so many other charms.

What makes this the best luxury super-sedan available is the quality of its mechanical core. There isn’t a BMW that can match it for driving dynamics, or a Mercedes that feels more impregnable and impressive. At the moment, this is as good as it gets – at least on the showroom floor.

Then try…

Big sedans like the Mercedes-Benz S-Class take hard valuation hits in the used car market. And that’s particularly true of its performance variants.

Mercedes’ 2007 S65 AMG, powered by a twin-turbocharged 6-litre V12 engine rated at a mind-scrambling 604hp, carried a $181,500 price tag when it was new, and thundered from a standstill to 60mph in about 4.5 seconds. Today, according to the car-valuation website, that same car would run about $47,500 from a dealer. Sure, that car is likely to have around 80,000 miles on the clock, but its owner has swallowed a staggering $134,000 in depreciation.

Forget maintenance, fuel consumption, insurance and regular detailing expenses for a moment. That S65 AMG has been burning bucks at $1.68 per mile. And that terrifying drop in value is only slightly less precipitous for the S65 AMG’s incrementally less muscular, 550hp brother, the V8-powered S63 AMG.

Like every S-Class, the AMG brothers are built on slow-moving production lines with an attention to detail that isn’t lavished on smaller, mass-market Mercedes models. Open a door and there’s a whoosh, as if a can of vacuum-packed coffee had just been opened. The front seats are thrones, the back seat is plusher than a Fortune 500 plutocrat’s couch and the trunk is so big it could host a professional sports franchise.

They are, however, massive cars; locomotives with license plates. They charge forward relentlessly and with little concern for things such as steering feel. These big, intimidating Germans squeeze pin-point sensation out of the driving experience and replace it with a confident, high-speed serenity.

Solid S63s and S65s are coming off of leases and showing up as certified pre-owned (CPO) cars at Mercedes dealerships around the globe. And there will likely be an oversupply in the next year as buyers trade up to the all-new, all-aluminium 2014 S-Class.

Also bolstering the AMGs’ appeal is the tendency of their owners to coddle. After all, if you can afford the car, you can afford to keep it in a garage and over-maintain it. That said, the big risk in any used German luxury car is that when it finally does break, the fix is seldom cheap.

But if you were considering an S8, you have already saved $70,000. Go forth and pay for that cylinder-head gasket with confidence.