I think the 6 series looks really bad with that big hump in the back. The car weighs too much as it is.
In all honesty I think that spoiler attachment over-does the car. In any case, it's definitely not worth 500 Euros. I also think the Audi logo on the side is unecessary, but hey - to each his own.
It looks good the way it comes with the rear spoiler.