I've been unimpressed with the sound on my 328i since I got it. I have the Harmon-Kardon system, and like many factory "premium" systems, it really doesn't deliver. Which is really putting it mildly. The bass is muddy, and the tweeters don't.
This weekend, I spend some time putting in a set of replacement speakers from Bavarian Soundwerks. I've probably replaced around 10 factory systems (sometimes speakers, sometimes whole systems) in my time, and the improvement this time is the most I've seen. Well, heard. The clarity and depth is so much better not to mention actually being able to *hear* the cymbals. The bass is tight (though not deep - you can only get so much from 6 1/2" drivers), and the factory amplification can produce reasonable (but not truly rockin') sound levels.  Apparently Harmon Kardon did a decent system design but BMW didn't want to spend the money for decent drivers.
If you've done stereo or speaker installations in the past, this is fairly straightforward stuff, but I'm not sure that I'd recommend it for a beginner. The instructions are pretty detailed (though they miss a few things, and my wiring and other stuff varied from the directions a bit), but you still have to pull off door panels, carefully pull out speakers, put the new ones in (some bolt in, some are installed with hot glue. (no, really, and it works quite well)).
The appearance is unchanged.
For replacing all 10 speakers in my system (four 6 1/2" bass drivers, two 2 1/2" midranges, and four 1" tweeters), the cost was around $500. You have a few options for drivers - the base system drivers are from CDT, but you can upgrade to MB Quart (my choice), or Rainbow for a little more money.
If you want the deep bass, they also sell easy-to-install subwoofers.
Definitely recommended.