The allure of a good grand touring car is pretty powerful. Just picture driving one through California's wine country while enjoying the car's sumptuous luxury, sleek design and copious comfort features. A BMW 6 Series coupe or convertible could fill the bill, of course. But so could the 2015 BMW M6, and it goes even further by providing sensational power capable of thrilling any driver.

The M6's twin-turbocharged V8 engine is good for a rather outlandish 560 horsepower and 500 pound-feet of torque. Nail the gas and you'll hit 60 mph in just a little more than 4 seconds. Achieving this is quite easy thanks to the M6's standard quick-shifting automated manual transmission, but BMW still offers a traditional manual transmission at no additional cost.

Other strengths for the M6 include a luxurious interior, easy-to-use technology features and lots of room up front. It's also extremely comfortable and surprisingly quiet -- an ideal combination for long road trips. A full-color head-up display, adaptive LED headlights, a 16-speaker sound system, sun-reflective leather in the convertible, and heated and ventilated "active" seats that work to decrease fatigue on long trips are just a few of the luxury items that contribute to a great driving experience.

As with all grand touring coupes, though, the 2015 BMW M6 does make some compromises in the way of practicality. The backseats are nearly unusable for adults and even cramped for small children, which is surprising given the M6's significant size. At the same time, its grand dimensions and hefty weight hamper its athletic aspirations. While the M6 is highly entertaining to drive, it feels much more at home on a long, gradual bend than a series of sharp chicanes.

Rivals include other high-powered grand tourers like the Jaguar XKR, Maserati GranTurismo and Mercedes-Benz SL-Class. The latter's retractable hardtop means you don't have to choose between a coupe and convertible. And while it's not strictly a grand touring car, the 2015 Porsche 911 shouldn't go unnoticed. It, too, is wonderful to drive, offers four seats and can be equipped in a variety of extremely lavish configurations.

All would make a grand-touring holiday (or even just the daily slog) a truly special experience, but know that the BMW M6 is an extreme version of an already great car.

The 2015 BMW M6 is available as either a coupe or convertible, both of which are offered in a single, very well-appointed trim level. A four-door sedan, the M6 Gran Coupe, is also available and it's covered in a separate review.

Standard features include 19-inch alloy wheels, adaptive suspension dampers, automatic adaptive xenon headlights, LED foglights, heated and power-folding mirrors, automatic wipers, front and rear parking sensors, auto-dimming mirrors and automatic soft-close doors. Coupe models come with a carbon-fiber roof, while the convertible features a power-operated fabric top and a glass rear window that can be raised independently of the top to serve as a wind blocker.

On the inside you get dual-zone automatic climate control, keyless ignition and entry, leather upholstery (sun-reflective leather seats for the convertible), 14-way power heated front seats (with four-way lumbar support, adjustable bolsters and memory functions), a power tilt-and-telescoping steering wheel, and extended leather and carbon-fiber interior trim.

Electronic features include Bluetooth phone and audio connectivity, BMW's iDrive interface (includes a 10.2-inch display), a rearview camera, a navigation system, real-time traffic, voice commands and a 16-speaker Harman Kardon surround-sound audio system with satellite and HD radio and USB/iPod integration. Also included are driver-adjustable settings for the suspension, transmission, steering and engine in varying degrees, from comfort to all-out performance.

An optional Competition package provides a freer-breathing exhaust system that yields 15 additional horses; lightweight alloy wheels; revised tuning for the suspension, steering and rear differential; retuned stability control that provides a higher threshold for intervention in the M Dynamic mode; and a black chrome finish for the tailpipes.

The Executive package includes adaptive LED headlights, automatic high beams, a heated steering wheel, ventilated fatigue-preventing "active" front seats, a head-up display (available separately) and a power rear sunshade (for the coupe only). The Driver Assistance Plus package includes a lane-departure warning system, frontal collision warning, a blind-spot monitor and a camera system with top-down and around-the-corner views.

Other options include various 20-inch lightweight alloy wheels, carbon-ceramic brakes, a premium 16-speaker Bang & Olufsen sound system, enhanced smartphone integration and a night vision system with pedestrian detection.

Powering the rear-wheel-drive 2015 BMW M6 is a twin-turbocharged 4.4-liter V8 that produces a prodigious 560 hp and 500 lb-ft of torque. An available Competition package gives the M6 an additional 15 hp. A seven-speed automated manual transmission with shift paddles is standard and a traditional six-speed manual is available as a no-cost option. An automatic stop-start engine function to conserve fuel at stoplights is also included.

In Edmunds performance testing, a convertible M6 with the auto-manual transmission sprinted to 60 mph in a mere 4.2 seconds. We expect the lighter coupe to be marginally quicker.

The EPA estimates fuel economy for both the coupe and convertible at 16 mpg combined (14 city/20 highway) for the automatic, and 17 mpg combined (15/22) for the manual transmission.

Standard safety features for all 2015 BMW M6 models include antilock disc brakes with advanced standby and drying features, hill-hold control, traction control, stability control, front-seat side airbags and active front-seat head restraints. Convertibles receive added rollover protection.

Standard on both the convertible and coupe is a rearview camera integrated into the 10.2-inch touchscreen display. Also standard is the BMW Assist emergency communications system (provides automatic crash notification and on-demand roadside assistance) and BMW Remote Services (adds stolen-vehicle recovery and remote door unlocking). The optional Driver Assistance Plus package includes a lane-departure warning system, a blind-spot monitor and a camera system with top-down and around-the-corner views. Also optional is a night vision system with pedestrian detection.

In Edmunds brake testing, a convertible M6 came to a stop from 60 mph in 108 feet, an average distance for this class of car on summer tires.

If you're interested in the 2015 BMW M6 because of the atomic power provided by its twin-turbocharged V8 engine, you definitely won't be disappointed. In its sharpest performance settings, this car is a fire-breathing beast that snorts and snarls with uncommon aggression. Yet the acceleration won't terrify you, as it's accompanied by a refined stability control system that modulates power delivery without shutting down all of the fun.

What we've found surprising, however, is that the M6 is a joy to drive whether you're just leisurely cruising down the highway or stomping on the throttle with commitment. Thanks to adjustable settings for the suspension, steering, transmission and throttle response, the M6 can adapt to your every mood.

Even so, this M6 is a heavy, grand-touring-oriented machine. Once you add the poor sight lines over the long nose from the driver seat, it becomes clear that this car is at its best when the road is fast and open. The M6 is a very fast grand touring car, not a sports car.

Just as you'd expect of a six-figure BMW coupe, the 2015 M6 easily fulfills the promise of a top-shelf luxury experience with high-performance elements. Rich materials adorn every interior surface and expert craftsmanship is evident throughout. The front seats feature an abundance of power adjustments to ensure either a comfortable fit or racecar-like lateral support for those moments when you're driving the M6 hard. Rear seat passengers will not fare nearly as well, as the cramped space is suitable only for small children. At the same time, just about every other competing coupe or convertible is similarly confining in back.

BMW's classic analog gauges are easy to read at a glance. The iDrive interface works well for controlling and adjusting all of the M6's systems. In our experience, though, iDrive typically takes a few more clicks and twists of the control knob to get what you want; some rival systems are easier to use.

Unlike some other convertibles in this price range, the drop-top M6 won't require much in the way of functional compromises. With the top up, the well-insulated convertible isn't much louder than the coupe. With the top down, the separate glass rear window can be raised to provide an effective wind blocker, permitting civilized conversations and preventing a bad hair day. Also noteworthy is the useful trunk, which measures 12.4 cubic feet (10.6 cubic feet with the top retracted). The coupe's trunk holds 13 cubic feet.