THE Mercedes-Benz SLK makes some tempting propositions, many of which obviously prove irresistible for car-buyers in this segment. It is, for a start, the most user-friendly of all "proper" convertibles. It also has the appeal of a muscular, supercharged engine to ensure it escapes any "hairdresser's car" connotations. The SLK is also a true Benz in its solidity, fine ride, handling and high levels of safety. And it can be driven out of the showroom for considerably less than a Boxster.