I have one - its fantastic.
Its like a GTI but bigger - Seriously. I went from a mk5 GTI to a S3 then this. The S4 has the same ability that the GTI had to be a mild mannered family wagon or a nutter depending on your mood. The S3 was just dull. Technically very good but no soul at all.
Over its first 4000 miles the average MPG is 27 mpg. You can easily crack 30 on a longer journey with no town traffic and a bit of spireted driving will see low twenties.
Power delivery is very linear. From a standing start the acceleration is brutal, but oddly it doesn't
feel as fast if you put your foot down when rolling along - but the speedo moves plenty quick - it can be a shock sometimes to see how fast you're actually going. Oh and the fart on upshift at full chat is mighty addictive.
Still keep an eye on whats happening here from time to time