The BMW Z4, which replaced the Z3, has shaken if not rocked the sports car world, mostly with its dramatic styling. But a potential buyer can use his or her eyes, make his or her decision and move on to the objective qualities. Because sports car buyers are enthusiasts, those would be engine, transmission, ride, handling and brakes. The Z4 excels in all of those areas. Interior comfort and convenience are also important qualities, and the Z4 scores high in comfort and nearly as well in convenience, considering the quality of its soft top with glass window and defroster; it's easy to operate manually (standard) and if that's too much trouble, one finger will raise and lower the available power top.