I have a problem with these super saloons (such as M5s, RS8s etc not just the Cadalliac). They're cool and everything but they'll always be a compromise.
They're not super cars because they don't have the drama, exotica or puriety of design of a thoroughbred supercar (like a Mclaren F1, Zonda, Ferrari, Lambo, Porsche etc).
They also fail as a daily commute, comfy-sofa, pratical driver due to the agressive suspension, wheels, engine etc.
They're neither one thing nor the other.
So what are they for ?
If I had loads of money to get a car I'd buy something simple and inconspicious (like a Ford Focus) to get around in and buy something totally lunatic (like I dunno, a Lotus Exige?) for track days, high-days and holidays.