Counterpoint: George Achorn / Current Vehicle: 2000 Volkswagen Golf 1.8T
One of the best aspects of the S4 Avant is that it goes so against the very nature of the SUV establishment. While the older A4 platform is on the small side, and this is most apparent for back seat passengers, it's about as roomy as the smaller contenders out there in SUV land.
As a self-proclaimed fan of most VW products, I find the 2.7-liter turbo equipped in the S4 to be one of my favorite engines - period. The smooth and ample power delivery is intoxicating, and coupled with Audi's truly capable Torsen-based quattro system it is very easy to become addicted to.
The main problem I had with the car was shifter shock. I picked up our S4 test car from the Washington motor pool as I was returning an Audi TT quattro roadster. While the S4 was the clear superior to the TT in acceleration, it's excrutiatingly long shift throws felt almost truck-like as I switched over from the TT and it's very aggressive throw pattern.
Counterpoint: Jamie Vondruska / Current Vehicle: 2001 Volkswagen Jetta 1.8T
The S4 Avant is my favorite S4 model, combining the best of stealth grocery-getter looks with seamless power and wonderful handling - a vehicle for all seasons and all reasons. As Paul pointed out, only the hard-core enthusiasts will spot this special S-model Audi, and the fact that the wagon version is fairly rare, makes it even better.
Audi has become a master at producing cars that handle well, yet don't beat you up with poor ride quality. Attention to fit, finish and detailing is Audi typical first rate and creature comforts can be found everywhere in our heavily optioned test car. Power is extremely linear and pulls strongly from down low without much indication that twin-turbos are lurking under the hood. Add all-wheel-drive to the mix and I think you have one hell of a car.
There are a few nit-picks though. Interior room is very tight particularly for rear seat passengers and headroom for my 6'0" frame was pretty limited with the sunroof cutting into available space. I'll echo the comments about the sloppy shift mechanism as well, something I've noticed in every S4 I've driven.
Outside of those few minor issues, the S4 Avant is in my mind the ultimate wagon providing power, performance and restrained looks unavailable in any other package.