The Harmon Kardon system, is not bad, but it's FAR from great.
I'm putting together a system for my 2002 M3 that should be great.
The best part about it, is that I will still have my trunk space when I'm done, but it will be a 1000 watt RMS system.
Total cost of the system will be just over $8,000, it should be done in about a month. When it is, I'd be glad to show it to you.