There's plenty of substance to the 2015 BMW 6 Series, but it's style that distinguishes these driving machines. The most striking lines belong to the 2-door Coupe and 4-door Grand Coupe models with their sleekly arched tops that give off an intriguing mix of confidence and luxury. The convertibles sacrifice those rooflines for wind in the hair, but all three of the 6 Series bodies share the long hood marked by a pair of creases that lead to an aggressive front end with wide-mouth air intakes, LED foglights and the trademark twin-kidney grilles; flared fenders and a low-slung side crease; and an elegantly simple rear end that completes the overall aesthetic.

Now in its third generation, the 6 Series offers a few new tweaks across its lineup for 2015. There's an updated instrument cluster display. A touchpad is now integrated into the iDrive infotainment dial control. A new Launch Control feature comes with the 8-speed automatic transmission, the standard Harman Kardon sound system has been upgraded, and the 650i models come with an M Sport exhaust system.

The 6 Series also gets the second BMW Alpina model available in the U.S.—the Alpina B6 xDrive Gran Coupe—which made its debut at the 2014 New York Auto Show and joins the Alpina B7 as one of two vehicles to come from Alpina, an independent tuning shop that works in close conjunction with BMW. Some of the unique Alpina B6 xDrive Gran Coupe features include a modified powertrain for increased speed and acceleration, larger brake discs, 21-inch multi-spoke wheels, custom body kit, Alpina logos inside and out, and unique leather interior trim.

The 640i models (Coupe, Gran Coupe and Convertible) are powered by a 3.0-liter turbocharged inline 6-cylinder engine (315 horsepower, 330 lb-ft of torque) that gets 22 mpg city/33 highway in rear-wheel drive coupes, 20/29 mpg in all-wheel drive (or xDrive, in BMW parlance) coupes and convertibles, and 20/30 mpg in a RWD convertible. The 650i models come with a 4.4-liter turbocharged V8 (445 horsepower, 480 lb-ft of torque) that gets 17/25 mpg when paired with RWD and 16/24 with AWD. All these models come with an 8-speed sport automatic transmission. The Alpina B6 xDrive has a high-performance version of that 4.4-liter V8 that churns out 540 horsepower and 540 lb-ft of torque, goes from 0-60 mph in just 3.7 seconds, has a top speed of 198 mph and gets 16/24 mpg. The Alpina 6 also has a modified 8-speed automatic transmission and suspension.

All 6 Series cars come with Driving Dynamics Control, which allows the driver to shift between Comfort, Sport and Sport+. The different settings adjust the suspension, steering, throttle and transmission. These options, along with the powerful variety of engines and other handling technologies like Active Roll Stabilization, help the 6 Series deliver a memorable driving experience that can range from a luxurious cruise to a road-gripping tear. There's also an Eco Pro setting that allows for maximum fuel efficiency.

Thanks to the 10-way power front seats (20-way in 650i models) with four-way lumbar support, the driver and front passenger will have no problem finding a comfortable seat in the 6 Series. The rear seats, however, are a little cramped, especially in the 2-door Coupe and Convertible. The Gran Coupe offers a bit more room in the back, but it's still best-suited for a maximum of 2 passengers.

The lack of rear-seat room, and a less noticeable lack of headroom, are the only complaints about the interior. The materials—wood, leather and soft-touch plastics—vary with each trim, but are all high quality. There's an abundance of comfort technology, but it doesn't clutter the cabin. Standard features for the 640i models include auto-leveling xenon headlights with retractable washers, power heated mirrors, heated front seats, dual-zone automatic climate control, navigation with a 10.2-inch display, ambient lighting, dynamic cruise control, rear-view camera and a 9-speaker, 205-watt audio system. The 650i standard upgrades include 19-inch wheels, Nappa leather upholstery, a 3-year subscription to Concierge Services with Critical Calling and a 16-speaker, 600-watt Harman Kardon Surround Sound audio system.

The optional equipment list is lengthy with a focus on packages—Driver Assistance Plus (front- and top-side cameras, lane-departure warning, low-speed collision prevention, frontal-collision warning and speed-limit information), Cold Weather (heated rear seats and heated steering wheel), Lighting (LED headlamps and automatic high beams), Executive (head-up display, four-zone climate control, ventilated front seats, soft-close doors and a premium sound system) and M Sport (most of the Executive features plus 19- or 20-inch double-spoke wheels, lower body aero styling and simulated-suede headliner). Some of the individual option highlights are adaptive cruise control, night-vision camera, automated parallel-parking assist, Adaptive Drive, Active Steering and a 1,200-watt Bang & Olufsen audio system with 16 speakers (each with its own amplifier), a must for any audiophile.

All 6 Series models are loaded with standard safety equipment including dynamic stability, traction and brake control, cornering brake assist, automatic brake drying, antilock brakes, start-off assistant and a full array of airbags. Front and rear parking sensors are also standard across the board, as is the BMW Assist emergency system (automatic crash notification, stolen vehicle recovery, roadside assistance and remote unlocking).