The 2018 BMW M550i xDrive arrives on a barge. Bathes you in luxurious leather. Accelerates to 60 in less than four seconds and continues to pull well into the triple digits. Plays an interior exhaust note that would make a hot rodder or an Italian aficionado proud. And employs sophisticated electronics and an active suspension to lend its driver an air of invincibility. I pine for a super car. Is this one? Read on and decide.