I have a 2004 cabriolet with 87K and 2 owners. Running costs are not excessive but then again only use it for weekend/occasional driving. I am a complete OCD freak so I have probably spent too much on mine making it as fresh as possible. Mpg and insurance are more than reasonable for my estimated 4K per year. Mines a manual and had a new clutch/flywheel before I bought it at a cost of £1,300 and I have to say its a lovely drive, good positive shift and nice light clutch action. The overall performance and driving experience is pretty good if not exceptional in this modern world of lighter and powerful turbo charged alternatives. Having said that the lazy torque filled drive or high revving aural pleasure make ownership worthwhile. Handling is secure and again very good for what is a heavy car. the interior and body feel like its made with no budget in mind as the quality is excellent. The sound is beautiful but if having to be honest and critical the pedestrians get to appreciate it more than the driver due to the usual high standards of Audi build quality from this era. The car feels like it could cross continents and go on forever. It has not been completely trouble free with a new ABS module and 'small' condenser fitted but I believe these are all 'age' related items that age dependant may well have been sorted on any potential purchase. Mine's now for sale but only because I can't justify having 2 cabriolets on the drive (just bought a new S3) despite my best efforts!
Ideally I'd hang on to mine so that should give you an idea of how good/bad they are. Go for it whilst there are still some good ones out there at reasonable money