What happens when Cadillac takes on BMW?

It must be hard to kick ass, decade after decade. It puts a giant target on your back, after all — you’re the one everyone else is trying to knock down, year after year.In the car world, Bayerische Motoren Werke AG has been that leader for a long time. BMW created the sports sedan segment with the M3, released an early luxury SUV with the X5, and is now focusing on the future with its "i" sub-brand. It’s even promised a self-driving car by 2021.Which brings us to Cadillac, an A