I tried E60 M5. It was sweeeeet, especially when you click on the M button.
Yebut - my RS4 gives me all 420 horses whether or not the S button is pressed, and I don't have to go through that truely hideous I-Drive bolloc&s. Oh, and did I mention, my RS4 (even my old S4) will quite literally run rings around ANY BMW, even the "M"s, with the lightest sprinkling of precipitation!
I-Drive is not my favourite, either. But I doubt S4, and even RS4 will run rings around 507bhp M5.
BUT, if I had the chance to choose between M5 or the latest model RS4, I'd choose RS4, it looks better.