to be honest, I've driven the IS-F on the track. I liked that car quite a bit.
I wanted to hate it before driving it, but I can't speak anything bad about it. it was quite a performer. the transmission was very crisp and had no delays--it was great, but I think 8 speeds may be too much. the exhaust note was perfect. the car pulled very well and could hold its own on the track.
the only reason why I wouldn't get it over an M3 is b/c I think the M3 looks far better inside and out and the IS-F is a four door--obviously a very subjective matter. also, I think I prefer higher revving engines and from my short drive of the e92 M3, I think the steering was more crisp and it provided better feedback. but, if the e92 M3 wasn't a consideration, I wouldn't hesitate getting the IS-F over the C63 (which has a terrible transmission).