Jalopnik had a chance to drive the Lexus IS Convertible, and their biggest complaint was the lack of rigidity in the body:
The ‘vert gets beefed up in nearly every way, with side sills 50% taller, cross-vehicle beams under the rear seats and behind the rear seatbacks, a heftier A-pillar and substantial cross bracing underneath…Despite all the reinforcements, the folly of adapting a closed car chassis for convertible duty is apparent in a bit more open-top cowl shimmy over rough pavement than we’d expect in a Lexus product.
Much like every other Lexus on the market, this new convertible doesn’t really have any glaring faults. Unlike the sedan, the IS C is a competent grand tourer, but not a sports car.
By its very nature, a convertible is not going to be as stiff as a sedan or fixed-roof coupe, and some flex in the body has to be expected. While it’s true that the IS convertible is built off the sedan chassis, the roads must have been exceptionally rugged for the ride to be so rough.