I loved the setup, full cat-back pipes. It sounded better the longer it was on the car. When I ran it in Targa Newfoundland, it was touted as one of the best-sounding cars in the rally. It can get loud in the cabin but never droning or obnoxious. The engineering and design of this car are so good that isolation from outside noise was never an issue for me. And when you stomp on it, it just sounds plain wicked. My inspection guy claimed they were a bit too loud but never failed me, but he was a bmw man himself.
Here is the kit when I first installed (before the CNS clutch, btw...you'll hear the stock clutch slip a bit)
e39 M5 CNS exhaust drive away - YouTube
This is the car running a stage in Targa so you can hear what it sounds like inside the car.
Targa Newfoundland 2010 M5 on-board Brigus - YouTube