I had what I hope was a unique experience with a 2004 S4. Failed leftside variable camshaft timing adjuster at 4,200 miles. Car wouldn't idle correctly due to the loss of valve timing control at low rpm. Long story short - car was in shop for over 2 months, engine was R&R'd twice after long wait for parts from Germany. Audi finally decided, with some serious prompting on my part, to replace the car with a new S4. I've had it for about 7 months now with no similar, or other, problem.
Hopefully it was a fluke and not a common problem. The longer I have the S4 the more I like it. Really fine design, engineering and execution.