Have not driven S4 but I think it's one classy and sexy ride. Not over the top, but with nice luxury. Can't really be compared to Suby. Everything Impreza screams cheap inside (love the exterior), but then it's not a car for luxury. Two different vehicles targeting different demographic...
Having said that, I have a Q7 for family hauler that is infested with problems, and it has just over 30k miles.. Bulbs blew out out of nowhere countless times. Water entering the cabin due to stupid drainage design. Just few days ago, an amplifier went out turning off MMI completely (rear camera, navi, ac, heater all not functioning). Never have I been so glad I bought extended warranty (car is 2009) - but I'll be getting rid of that car well before the warranty expires for the next owner's sake.
With these problems, there still are enough things that I love about Audi for me to not hate it. Fantastic and ergonomic interior, technology and convenience is amazing and exterior looks great.
Bottom line is that... you'll never know unless you live with it. Different people with different purpose for a car. Performance isn't everyone's top priority.
aaaand I want a GD STi for the winter