This will depend on how good the stock speakers are in your car, of course.

The stock speakers in most cars are just begging to be replaced, though.

The main thing that I noticed when I replaced the front door speakers in my Honda with Polk Audio coaxials is that I was no longer hearing the speakers any more, now I was hearing the music. I know that sounds like salesman-speak, but it was true in my experience. The tinny high-midrange resonances in the stock speakers were just too overt, and made the system sound cheap. It drew attention to the location of the speakers so that I was acutely aware of their locations in the doors. After the polks went in, I was no longer distracted this way because the frequency response was smoother and flatter, without unnatural peaks.