I've owned my S4 for over 30k miles now, so I think I can properly review it as a used car. It's a blast to drive if you drive it like it wants to be driven. You honestly have to take care to drive it slowly. I commute 3 hours round trip to DC everyday and while I hate my commute, if I just stomp on the gas in 3rd gear at 3k revs, it makes you smile like a teenager watching American Pie during a certain scene. Gun it in first and you almost feel uncomfortable. It throws you back and takes some getting used to. I typically drive conservatively, but if you downshift in this car and hear the growl, it just makes you smile. I give my car to my wife when I'm out of town, and it scares her.
I bought this car when I was home on R&R leave from Afghanistan back in February. I had been researching the A4 for about 7 months and had every intention of buying that once I got to the dealership. As the Carmax dealer was walking me to the A4's I saw my knight in shining armor out of the corner of my eye. The girl opened the driver's side door and I was sold. I didn't even test drive the thing, and it was a manual so I had to teach myself, but trust me it was WELL worth it. I took a look at the A4's once I was done drooling over my soon-to-be new car and there was absolutely no comparison; the A4 looked like a grandma car compared to what I was about to roll home in!
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!
Thanks to most of the reviewers here, I made the decision to purchase a used S4 (along with Audi's pure protection additional warranty). I have to say it is the most amazing car I've ever owned--even over my 67 Mustang fastback. As another reviewer said "it does everything right". A pleasure to drive, doesn't look fast but runs like a refined rocket, handles like a track car and is great in snowy terrain of the north west. Highway MPG is doable, and for those that lament about city MPG, realize it's 340HP w/over 300lb ft torque...in the same engine compartment as the A4 (which I've owned 1.8t) and also liked. This is like a supermodel love affair in comparison. Gracious, powerful, refined
I previously wrote a review on my car at about 70k miles and she was still an awesome car (I bought her used in 2007 with 28k miles). I say still because at about 84k miles my timing tensioners started to fail and my timing chain had the infamous rattle at start up. At first, not a big deal, but the check engine light came on and the rattle became worse and worse. I knew the repairs would cost thousands, so I had to get rid of her. It was bitter sweet because I got a new S4, but I miss the V8. The suspension isn't meant for bad roads, so beware of suspension work if you live in a city of anywhere the roads aren't great. That was several thousand $$$ too.