For 2013, the BMW X5 M gets standard "multicontour" front seats and the brake calipers have been painted blue. LED headlamps are now optional.

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.

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.