Its DECENT, but can never replace high quality aftermarket stereos. HK sounds awesome in the BMWs, but seriously lacks bass in some models. 7 series sounds good.
Sounds pretty good in the E class too, however, it has "idiot controls" IE, cuts out the bass at high volume so the speakers dont blow, etc.
They are good, but nowhere worth the huge money that the upgraded system costs from the factory... You can spend under 1000 dollars like me, and have sound that trounches a OEM harmon kardon sound system.