I shopped the IS with the A4 and the 3 Series. Didn't like the way both Germans' headrests actually touched back of my head in normal driving position. The Lexus trunk was wider, better able to take 2 golf bags. I really like the styling of the IS. The build quality is good but not great. My car did have the center dash/nav rattles that others complained of but my local dealer seemed to fix it. I am disappointed with the way the car drives. Mine has sport susp. and tires. Handling is great but the ride is too harsh. Sport tires VERY road-noisy. The steering is the major flaw. Very fast and tight, but no road feel or feedback. It feels "sticky" and "dead." Very strange. Brakes grabby.
Great styling, good gas mileage (premium gas) comfortable front seats. Lexus service is first-class.
More back seat room (and under-seat toe room!!) More attention to build quality/rattles. Better stereo. More steering feel, less road noise. Sexier engine note.