If you're looking for a logical reason to buy a 2014 BMW X6 M, you'll probably be searching for a long time. That's because there really isn't one, as this ultra-performance version of the BMW X6 crossover gives a big, fat raspberry to the rational car-buying process. But if this unusual mix of crossover SUV, coupe and supercar appeals to you on an emotional level, well, you'll find it's like nothing else.

The brashly styled X6 M augments the standard model's sweeping fastback roof line and bulging wheelwells with gaping front air inlets that look ready to ingest anything that dares to cross the vehicle's path. A 555-horsepower turbocharged V8 engine backs up the menacing facade, and it's able to motivate the M's considerable mass with ridiculous ease. This marvel from BMW's M division also does a fine job going around corners. Of course, a tight road will quickly give you a sense of paint-scraping anxiety as the bulky X6 passes perilously close to shrubs and canyon walls, but the capability for making time on a winding road is certainly there.

But you are buying an SUV, right? Well, sort of. The standard configuration on the X6 M only comes with four seats, and your backseat passengers will find limited headroom because of the coupelike roof line. Moreover, the X6 M's cargo capacity is pretty meager and isn't well suited for hauling bulky items. The supercharged V8 2014 Land Rover Range Rover Sport and 2014 Porsche Cayenne Turbo are far from being practical purchases, but these sport-tuned SUVs are certainly more useful for families than the X6. And if style and performance are really your top desires, the Audi RS 7, BMW M6 Gran Coupe and Mercedes-Benz CLS63 AMG deliver something truly different from the norm and are far more coupelike than the X6 M.

So there's a strong logical case for buying something other than the 2014 BMW X6 M. But maybe you simply like the idea of owning a 555-hp BMW, and everything else is secondary. At that point, picking up an X6 M is totally OK by us.

The 2014 BMW X6 M is a performance-oriented version of the BMW X6 midsize crossover SUV. Four seats are standard, and an optional rear middle seat increases capacity to five.

Standard equipment includes 20-inch wheels, performance tires, adaptive dampers, active stabilizer bars, a self-leveling rear air suspension, adaptive bi-xenon headlights, auto-dimming mirrors, a power liftgate, front and rear parking sensors and a sunroof. Inside, you get dual-zone automatic climate control, heated 14-way power front seats (with adjustable bolsters and lumbar), leather upholstery, the BMW iDrive electronics interface, Bluetooth connectivity, a navigation system and a 16-speaker sound system with a CD player, HD radio, an auxiliary audio jack and an iPod/USB audio interface.

The Cold Weather package adds heated rear seats and a heated steering wheel. The Active Ventilated Seat package adds ventilation and massage functionality to the front seats. The Rear Climate package gets four-zone automatic climate control and manual rear side sunshades. The Premium Sound package comes with satellite radio and an upgraded 16-speaker sound system. Choosing the Driver Assistance package adds a head-up display, rear- and top-view parking cameras and automatic high-beam control.

Stand-alone options include keyless ignition and entry, automatic soft-close doors, a sideview parking camera, LED headlights, extended interior leather trim, a rear-seat entertainment system and the BMW Apps suite of iPhone integration features.

The 2014 BMW X6 M is powered by a twin-turbocharged 4.4-liter V8 engine good for 555 hp and 500 pound-feet of torque. A six-speed automatic transmission and all-wheel drive are standard.

In Edmunds performance testing, the BMW X6 M reached 60 mph from a standstill in just 4.3 seconds. This makes it not only the quickest SUV we've ever tested but also quicker than many dedicated sports cars.

The trade-off for so much performance is fairly dismal fuel economy -- the X6 M returns an EPA-estimated 15 mpg in combined driving (13 city/17 highway).

The BMW X6 M includes stability and traction control, antilock brakes, automatic brake drying, front side airbags and side curtain airbags. Also standard is the BMW Assist emergency communications system, which provides automatic crash notification, stolen vehicle recovery and on-demand roadside assistance. Optional safety equipment includes sideview, rear- and top-view parking cameras.

In Edmunds brake testing, the X6 M came to a stop from 60 mph in an exceptionally short 107 feet.

The 2014 BMW X6 M is a crossover SUV that weighs more than 5,000 pounds. So how does this beast manage to handle like a sports car? Technology -- lots of technology. There's "Active" this and "Dynamic" that in just about every area of the X6 M's operation, a battalion of high-tech aids designed to keep the laws of physics at bay. The results are stunning. Drive the X6 M enthusiastically through a turn and you'll experience neutral handling, prodigious grip and an athletic character. However, on really tight roads, no amount of athleticism will be able to hide just how big the X6 M is.

Then, there's the turbo V8 engine. Power is instantaneous and acceleration is at once awe-inspiring and giggle-inducing. Press the driver-programmable M button on the steering wheel and the gas pedal becomes more sensitive, the automatic transmission's shifts become more rapid and the exhausts let out a gnarly bark during upshifts under full throttle. The X6 M may not be a classic performance machine, but it's certainly a riot.

Besides unique M additions that include special seats, badges and color combinations, the 2014 BMW X6 M seems quite familiar. From the front seats, it looks nearly identical to the previous-generation X5. Beyond that, the materials are top-notch and the whole thing is put together beautifully. The iDrive interface works well for controlling and adjusting all of the X5's systems, though in our experience it typically take a few more clicks and twists of the control knob to get what you want; some rival systems are easier to use.

The coupelike roof line is, of course, the whole point of the X6, but it does reduce rear headroom in comparison to other SUVs. In standard form, the rear seat has a large center console and accommodates just two passengers; you can opt to have a three-person bench instead. The outboard rear seats are reasonably comfortable but they lack any sort of adjustment.

Besides limited backseat headroom, the dramatic roof line also reduces visibility and cuts into the cargo area. Cargo capacity is sufficient for occasional hauling, with a decent 25 cubic feet with the seats up, but with the rear seats stowed, the X6 holds only 60 cubic feet. That's less capacity than you'll get out of a Ford Escape. Then again, Escapes aren't pumping out 555 hp now, are they?