The ISF was the better sports sedan compared to the BMW M3 and the Mercedes Benz C63 of the same era. These cars were fragile compared to the Lexus which was robust in a typical Toyota engineered way.
In 2007/08, ISF was breakout in Lexus/Toyota, also was the completely new chassis, engine, and improvised transmission with limited budget and the top-tier Toyota engineers masterpiece. Without the creation and success of the ISF, there would haven’t been a LFA, GSF, and RCF later on.