I studied Acoustical Engineering and can tell you its a great system for a few hundred pounds. The clarity and balance is excellent.
The main 'complaint' that people have is it is not loud/bassy enough - I assure you that unless you have a hearing problem it is plenty loud enough!
It goes up to 34, and the loudest I have it is about 28 - if the Mrs is in the car she gets grumpy over about 23.
The B&O has the 'right' amount of bass - so if you are into Drum n' Bass then maybe the Audi system would fit that type of listener better - but I listen to a wide range of music (including some Drum n' Bass) and it sounds just fine to me!
The Audi system that comes as standard is pretty good, but its no B&O!