Currently, I own a Hyundai Tiburon with the Infinity system from the factory.  I've always thought it sounded good.  My previous vehicle, a Nissan Sentra SE-R SpecV, had a Rockford-Fosgate system which I think was superior to the one I have now.  Both systems cost about $500-$550 extra, dealer-installed.  I've heard custom install systems that blew my factory installed systems out of the water, but were comparatively very expensive.

Do you think factory-installed systems are a good value, or a waste of money?  Are similarly priced speakers bought separately a better option? Or would I have to spend a lot more money to get something better?