We bought this car used with 20,000 miles and have put another 90,000 on it. We also own a Mercedes CLK 320 and of course the S4 is more sporty, tighter and more fun.
however we expect our cars to last a long time (the CLK has 160,000 miles) and we're disappointed by the way the S4 has degraded. The doors rattle when cornering, it has a minor oil leak, the paintwork has not held up so well and any maintenance is labor intensive. It proably ends up costing more than the Mercedes. Cheaper initially but needs more upkeep. I'm not sure we'd buy another Audi. I suspect the brand has become popular due to marketing dollars, not engineering dollars.
More reliability and better build quality. Make it easier to repair.