The BMW 6-Series revels in its many personalities. As a 640i Convertible with all-wheel drive, it's an all-season, top-down cruiser. As the mighty M6 coupe with a manual transmission, it's a fearsome track-tackling juggernaut. And as a 650i Gran Coupe, it's a luxurious four-door that winks at practicality while it defies convention...