T-Bone is correct in saying that all of the competition have very similar performance with the exception being the C63 which is a little ahead of the chasing pack. And he is 100% correct in saying that the M3 is the class act when it come to finesse and dynamics, we do know that neither the RS4 or C63 are as capable on the track but we will have to see if the IS-F can surprise us all though based on early previews it doesn't look like it will bother the M3 is this department.
So on this glowing report of the M3 it would bag the question why is there a need for the rest of them but like gbb357 has already stated some people dislike the BMW brand, while other may not like the current styling and because of this there will always be a market for the rest of them.
And in any case each of the others aren't bad things to drive and each offer something different to the BMW, Mercedes has more performance and the image, Audi has all weather performance and the Lexus has reliability and value for money.
At this price bracket there isn't really any bum buys.