IMO, there are a few design curves of the IS-F that really ruin the whole thing. I'll do my best to explain......
Lets look at this picture:
Right below the taillights the rear bumper almost shoots up completely parallel. This might not seem like a big deal but, I'm a fan of symmetry. Now look right behind the front wheel, right above the GT-R slit. You see that design line..... Now draw an imaginary line to the back. Very sloppy. Also, it seemed that they completely forgot about that design line half way through the idea.
Also, what's the deal with that bump that comes out of nowhere right behind the rear wheels??? Again, sloppy.
Look at the lines of the rear door. Weird
Goofy exhaust. Again, IMO.
It was a good 1st attempt but, it's still seriously fugly.