Vomit

My car has the factory 6 disc changer in the glovebox, but no factory nav. system (most of you who have the factory nav. seem to hate it anyways). Does this mean that my car also has some sort of upgraded head unit/amp/speakers (i.e. "Premium Sound" or something of the sort)? The system actually sounds pretty good to me, but I play mainly chick music (80's pop and old school rap/jazz/swing). What is the main "weak point" of my factory sound system (and what would you replace it with). I would be especially interested in hearing from any of you who have replaced the head unit with the Pioneer or Alpine head units/navigation systems. The stock head unit is not perfectly rectangular. Were you able to make the replacement head unit fit cleanly?