If the 550i leaves you a little cold, you owe it to yourself to test drive a CTS-V. The V is faster than the M3 (more than a second faster 0-100 mph), brakes quicker and is only one second slower through the slalom. Even versus the M3, several well respected authorities say the V is the better automobile:
http://www.thetruthaboutcars.com/com...cedes-c63-amg/
Oh and since you're considering an M3, have you seen the CTS-V Coupe?
Yeah, I know.
Want to read the review?
http://www.autoblog.com/2010/07/09/2...iew-road-test/
Nothing like a little honest competition to improve the breed.