Lexus seems to be slapping it on the cars with no regard to the rest of the design
I dunno, I think it works. The upper edge of the grille, before the line changes directions, echoes the edges of the headlights. The lower edge is reflected in the smaller vents to each edge of the front bumper. And overall, Lexus cars have been getting more angular and origami-like recently anyway. I don't think something less bold - say, the interim grille style on the CT 200h - would work with a design otherwise filled with sharp folds.
There is one other thing, and that's the fact that the grille looks less dramatic when there's a license plate stuck right in the middle of it, which is the case in many countries. Not visible in the images above obviously, but it does take the edge off it a little out on the road.
The other thing is that I quite like Lexus's design confidence. It's dramatic but I wouldn't go as far as calling it over-designed. Several other cars on sale at the moment have a sort of uneasy mix of sharp edges, curves, scalloped sides, odd details etc (I'm looking at you, 1-Series) - to me, the IS is at least consistent across the whole car (those odd headlights aside).
I quite like the rear lamps too, AMCNUT! It's a very concept car-style touch. Looks like the lights are peeling out of the bodywork between the metalwork and the rear bumper. I'm a big fan of the GS, but the IS has all of a sudden made the GS look a bit half-hearted.
But Lexus's current design language sits a lot more easily with me than many at the moment. It's like the car has been searching for a design identity since 1989 and they've finally found it in the last few years.
Maybe I'm just odd.