Can't say enough good things about my M3. I love it! I have test driven many of these over the years, and I finally took the plunge and bought one earlier this year. It is everything I have always wanted in a car. Excellent good looks, enough room to have my wife and two kids in the car (plus a decent amount of luggage in the trunk), and power and handling that are absolutely breathtaking. Even with the amazing raw power that the engine is capable of, it still gets very good mileage if you aren't "on it" all the time. When not driving aggressively, I have averaged 27.1 mpg for a whole tank of gas. The ride is very stiff, so if you are looking for a cushy car, this is not the ride for you.
7900 rpm redline. I keep thinking "I should shift now...", but then I realize I'm only at 6K rpm and I have almost 2K more to go. :) I also love the rawness of the power in this car.
A little smoother ride under normal driving conditions that only stiffens up when you're driving aggressively. I can't think of anything else that I would change.