Mercedes-Benz is on a roll. After the excellent new C-class and E-class saloons – proper Benzes, as opposed to BMW wannabes – now comes the most eagerly waited new sports car since… well, since last week’s launch of the Ferrari 458 Italia, and the previous month’s unveiling of the McLaren MP4-12C, and the previous month’s brilliant new Porsche 911 Turbo. So much for the prophets of gloom,