Hey a friend of mine was saying that the Audi RS4 will kill an M5 in acceleration, top speed, and handling around the track. I have driven a heavily modified s4 and really enjoyed it. I drive a sc 540/6 which is quicker than a m5 but not as refined. I like the Audi very much and would not hesitate buying one BUT though they are quite good values, fast, and fun to drive they just don't feel the road like a nicely modified 5 series. The s4 is not as tight in construction when parked next to each other the bmw hkas SO much more style. If speed is your only benchmark get a Porsche turbo. He asked this same question at www.rs4.org and they were saying that the RS4 is soo much better.