I know I may sound 'old-fashioned' but I have never before tried to change the sound system in my current car as well as those that I have previously owned. The pre-installed one just suits me well. Anyone like me here?
My rule is if it ain't broke, don't fix it. If the sound system does what I want, then I'll leave it. If it doesn't, like it is only a radio and not a CD/MP3 player as well then I'll upgrade.
What's wrong with being old school? I, for one think that it's something to be proud of when you're not picky and if something works just fine and is enough for your needs then why change it?
I agree with Tadtone, why change it if there is no need to. Of course it's very rare to find someone who is perfectly happy with a system that someone else installed. I probably would want to tweak a few things here and there.
I have a friend who is so much into sound to the extent that he can spend a fortune on a car audio system. Since we don't spend a lot of time in the car, I see no need to spend money on modifications. If it sounds just ok, the better for me.