In terms of reliability, running costs and dealer reputation it comes out above all the rivals you mentioned.
I looked at C63, M3, RS4, XF-R & VXR8 before deciding on the IS-F, it ticks all the boxes that the others do and you don't hear many horror stories about them going wrong and costing thousands.
Can fit 4 adults in comfortably, boot is a decent size, capable of averaging mid 20's mpg (I've had 32mpg on a run before), excellent build quality, and can get 2 years manufacturer's warranty for less than £900.