To me the Harman Kardon system is very worth the upgrade cost. I enjoy a good sounding system and am willing to pay the relatively small cost for something I'm going to be listening to over an hour a day. I test drove a 230i with the base system, and while it wasn't bad, it was in no way impressive and would have left me wanting for better. The HK system is very good. Not the best, but good enough to be keep me from tearing the interior apart to replace components.
While the quality across the spectrum is better, the difference is most noticeable in the low-end thanks to the subwoofers under the two front seats. If you want to
feel any bass, you need to upgrade.
Regardless of all that, if you're worried you made a mistake you probably did. If you truly didn't care about the audio system, you wouldn't be worrying about it or asking for opinions.
