I've an 04 A4 2.5tdi Quattro sport. No real issues other than faulty ECU which was eventually replaced by Audi under warranty however I had to fight with garage I bought it from as they wouldn't pay as it run out of warranty while Audi had it but that's another very long story. Upgraded to bigger OEM wheels (18"), fitting cruise control, centre armrest and after market Veyron multimedia system which allows me to use my phone to play music thru it and also have satnav on at same time. Saves on battery life of phone if nothing else. Any mods I've done are down to car not having anywhere near the same spec as my previous C class MB however with it now being ten years old with over 100k miles on it the drive experience is still fantastic compared to the MB which was becoming very loose on the front end. Certainly not as expensive for OEM parts or servicing as MB and hopefully good for another 100k miles. Would certainly consider an A5 2 or 3 ltr coupe Quattro black edition tho, they seem particularly fetching so maybe change to one of them next year.