A 555-horsepower sport-utility vehicle with lots of sport and not so much utility can be explained this way: Why not? Sometimes it's impossible to explain our decisions, especially those concerning extravagant purchases. Why do the drapes operate by remote control? Why is there a Guinness tap in the kitchen? Why was the beef in this burger flown in from Kobe this morning? Why do you want a high-performance SUV that goes from zero to 60 mph quicker than an M3? Why not?

And this isn't just a matter of stuffing a twin-turbo 555-hp V8 into BMW's familiar midsize SUV and calling it a day. M division also took on the much bigger task of making a 5,300-pound crossover handle like a sport sedan. Performance tires are a good start, but it's the sport-tuned suspension with adaptive dampers that permits the 2013 BMW X5 M to out-handle any number of sport sedans and coupes. This is like successfully teaching a Yokozuna how to dance like Baryshnikov.

Despite this truly incredible degree of capability, though, there is literally a sizable problem. On the wide-open spaces of a track, you'll swear you're driving something much smaller. But since the tight, twisty roads of the real world tend to be rather narrow with ditches, drop-offs or canyon walls lurking perilously close to the fenders, it is impossible to forget the X5 M's substantial girth. Yokozuna may have the moves, but he'd constantly worry about errantly slamming Black Swan into the scenery.

Which brings us back to "Why not?" If you really wanted a weekend plaything for back roads, you'd be better off with a legitimate sports car or sport sedan. At the same time, a regular X5 xDrive50i is plenty quick and offers basically the same practicality. Yet such a vehicle just wouldn't be the same as a dedicated high-performance machine. There's a special type of pleasure derived from over-indulgence, and when it comes to SUVs, the 2013 BMW X5 M is one of the best ways to achieve it.

There are other ways, though. A Porsche Cayenne Turbo does provide a similar experience, but is $20,000 more expensive, while the Range Rover Sport Supercharged sacrifices some on-road prowess for go-anywhere off-road ability.

Are those better than the X5 M? Well, are platinum-plated kitchen fixtures better than 24-carat gold ones? It's hard to say, since it depends on your style, taste and just how over-indulgent you're feeling. Whichever one you choose, though, don't feel compelled to provide any more reason than "because I can."

The 2013 BMW X5 M is a high-performance version of the regular X5, which is covered in a separate review. The M version seats five people with no seven-passenger option.

Standard equipment includes 20-inch wheels, an adaptive and self-leveling adjustable suspension, adaptive and auto-leveling xenon headlights, LED halo running lights, auto-dimming and power-folding mirrors, automatic wipers, front and rear parking sensors, a power liftgate, rear privacy glass and a panoramic sunroof.

Inside you get keyless ignition, cruise control, dual-zone automatic climate control, heated 14-way "multicontour" front seats with four-way lumbar and adjustable bolsters, leather upholstery, a sport steering wheel, the iDrive electronics interface, a navigation system, real-time traffic, voice controls, BMW Assist emergency communications, and a 16-speaker sound system with a CD player, HD radio, an auxiliary audio jack and an iPod/USB audio interface.

The Active Ventilated Seat package adds ventilation and massage functionality to the multicontour seats. The Cold Weather package adds heated rear seats and steering wheel. The Rear Climate package adds four-zone automatic climate control and manual rear side sunshades. The Driver Assistance package adds a head-up display (available separately), rear- and top-view parking cameras, and automatic high beams. The Premium Sound package adds an upgraded audio system and satellite radio (available separately).

Stand-alone options include LED headlamps, a side-view parking camera, keyless ignition/entry, power soft-close doors, a six-CD/DVD changer, a rear-seat entertainment system, and the BMW Apps suite of Internet smartphone features. Leather trim for the dash and center console is a no-cost option.

The 2013 BMW X5 M is powered by a twin-turbo 4.4-liter V8 that sends 555 hp and 501 pound-feet of torque to all four wheels through a six-speed automatic transmission. In Edmunds acceleration testing, the X5 M went from a standstill to 60 mph in a staggering 4.5 seconds. Should you care, fuel economy is an estimated 12 mpg city/17 mpg highway and 14 mpg combined.

Every 2013 BMW X5 M comes standard with traction and stability control, antilock brakes, automatic brake drying, front side airbags, side curtain airbags and adaptive brake lights. The latter flash the taillights under sudden extreme braking as a warning to trailing motorists. Also standard is the BMW Assist emergency communications system, which provides automatic crash notification, stolen vehicle recovery and on-demand roadside assistance.

In Edmunds brake testing, the X5 M came to a stop from 60 mph in an excellent 116 feet.

In crash tests conducted by the Insurance Institute for Highway Safety, the structurally identical BMW X5 received the highest rating of "Good" in its frontal-offset and side-impact crash tests.

BMW's M division has made the X5 M astonishingly quick, and once you lay into the accelerator, you'll be at extralegal speeds in no time. Thanks to a bevy of high-tech bits, the X5 M also handles better than any midsize crossover SUV has a right to (and doesn't suffer a choppy ride because of it). There's "Active" this and "Dynamic" that in just about every area of the X5 M's operation, a battalion of high-tech aids designed to hold the laws of physics at bay. The results are truly astonishing. Even seasoned professional drivers will be amazed at this BMW's athletic character and prodigious cornering grip.

All that doesn't make the 2013 BMW X5 M a sports car, however. Despite its impressive ability to shrink around you on the open road and behave as if it were a much smaller vehicle, it balloons right back up to larger-than-life-size dimensions any time you drive on narrow roads. And since winding roads tend to be narrow, the X5 M tends to lose some appeal as a performance machine.

With the unique M sport seats and plenty of "M" badges throughout the cabin, the X5 M has just enough of a sporting appearance to make it feel as special as it performs. Otherwise, the interior is just as refined as the rest of the X5 lineup. The construction is precise, the materials are high-quality and the instrumentation is crystal-clear.

Most interior electronics features go through the iDrive interface. It's greatly improved from earlier iterations, provides a large degree of customization and allows the integration of Internet-based smartphone apps. However, operation remains complicated, and some rival systems are easier to use.

In terms of comfort, the X5 M has some of the finest front seats anywhere, offering an abundance of support and adjustment. Adding the optional ventilation and massage makes them even better. The second-row seats are mounted a bit too low to the floor, however; longer-legged passengers will likely bemoan this seating position, as it forces knees upwards. There's no third-row seat option as in the regular X5, but since that's barely usable, you're not really missing anything. The X5 offers 75 cubic feet of maximum cargo space, which is average for a midsize SUV.