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.
Engine growl, great transmission, steering, seat comfort, interior. It looks like a normal sedan, so don't expect stares. Buy the best tires you can buy. Seriously.
Everyone complains about economy, so just get used to < 20 mpg. It's like owning a SUV. Not much difference between 5th and 6th gear. I want an RS4 now.