Hi Mojo,
Yep, had the B & O in my old S4 and now have the BOSE in the S3, and to be honest, one of the main things I miss from the S4 is that stereo.
The B & O was superb in the A4/S4, giving a really rich, full sound across a wide spectrum of music.....by far the best stereo I have ever had in a car.
The BOSE suffers in comparison in my view....it is a good enough system, but it really doesn't have that depth of sound you got with the B & O, sounding really quite 'tinny' at times......and don't get me started on the automatic volume control which seems to have a mind of its own and drives me insane !!
That being said, I guess you can't really expect it to be as good, considering it has fewer speakers, is an older system with a more generic rather than a specific design for the car it is fitted to, and the my S4 was the saloon which I have always found provides a richer sound than hatchback's.
Like the others have said, there are several of us who would like to try upgrades to the BOSE, as I think it is a bit of a weak point in the 'feel' of the car.
Hope that helps.