[rcooled]

I had a '99 A4 2.8 for several years, now have an '05 S4 and have driven many other Audis ranging from the A3 to the Q7. Both the A4 and S4 are great driver's cars with the S4 having the obvious edge in performance. The 4.2 V8's 340hp will certainly keep a smile on your face...it's silky smooth and pulls hard to 7K rpm. It has a firm ride, even when compared with the sport suspension in the A4, but it isn't harsh. The S4's brakes are MUCH better than the A4's and it's Recaro seats are really supportive. Overall, it's about as you might expect...S4 = A4 on steroids. One downside is the drive-by-wire throttle which can seem a bit slow to respond at times. Another is the gas mileage...you'll rarely see anything better than 20mpg. Although mine has been trouble free so far, the S4 can be an expensive car to repair. An aftermarket warrenty might be worth a look. Audis are not DIY friendly, as Tim Hancock will confirm, so plan on using a good indy mechanic for anything beyond oil changes and minor tune-ups.
The 2.0 turbos I've driven had adequate power but all seemed pretty rough to me. People who like to modify their cars love the 1.8 & 2.0 'cause they're easy to tweak. The 3.2 is smooth and has good power but isn't nearly as exciting as the V8.
Control arm bushings wear quickly, so check to see if they've been replaced. Some of the Xenon headlight housings have a tendency to fog up in the rain but there's an easy fix for it. Coolant leaks and faulty coil packs are reported occassionaly, so check for repair/replacement. And lastly, cars built before 10/03 seem to exhibit more powertrain issues than those built after.