Never had the air conditioning serviced on 4 A6's(my wife has one too). The engine light has come on a few times, but I have always been able to drive fine until getting in for service in a day or so. Any problems we had were very minor, not affecting performance.
That's pretty much been my experience with my Ford. You're inevitably going to have problems. It's just a matter of whether you consider them significant. That's why I'm always a little suspicious of these polls that ask people whether they've had problems and they say "no." Do they really mean "no," or are they simply discounting the significance of the problems?
I've spent a lot of money having maintenance done on my car--brakes, belts, etc. All that doesn't count, though. It's to be expected on any vehicle.