I've owned my 2005 S4 since 2007 (bought used off lease at 24k miles) and it has been a great car. It is a lot of fun to drive with lots of torque and great V8 growl. Living in the city, I buy cheap tires (nails, etc.) and they still do great with cornering. I've had it at top speed (155 mph) and it was surprisingly smooth, although the revs were really high. Don't expect good gas mileage, but that's not why you buy thus car. It still looks sexy and I get compliments with a car that is almost hitting it's 8th birthday. I test drove the new S4 (2013) and it just wasn't the same. I have no idea what to get next!
Very very solid, reliable, comfortable seats, great engine, acceleration, torque to hurt your neck, exhaust note, fun - a lot of fun, actually. I've beaten M3's, 911's, WRX's, G37's... But, it's a great commuter too. If I have to choose, the torque and seats are my favorite. Steering is better than the V8 M3 in my opinion as well.
6th gear ratio, center arm rest (lifts with e-brake), larger gas tank.