I have been quite a car stereo afficiado over the years.
The stock stereo is actually fairly decent for a sports car. I have certainly seen worse.
Now....that being said, I have transformed mine into something unique!
The first question you should ask yourself is: What are you trying to acheive in a car stereo? Sounnd quality, quantity, options, functionality, capability.
If you are going to upgrade your system, things to consider:
If you replace the head unit, you need to replace the amp (run new wires, etc..most interior panels need to get taken off.) Now if you replace these 2 major components then you would be foolish not to replace the speakers which are the weaker stuff of the stock system. And if you replace the speakers....guess what? Yuo will need to replace the speaker wiring as well. Trying everything in the past, I can tell you that I learned early on that this is the best way to upgrade. After that, the skies the limit.
Here is what I have done in my 2001 ACR: Some say I am crazy, I like to call it different:
7" swivel widescreen; highest resolution on the market
Minidisc head unit with voice control, remote control, and DSP
10 disc CD changer
2 amps....over 1000 watts
8 speakers: 1 center, 2 tweeters in the door, 2 mids in the door, 2 coaxials in cargo cover, and 1 12" subwoofer
NOW IT"S GET TRICK....
DVD with optical outputs
Dolby Digital 5.1 processor and another DSP
Sega Dreamcast with wireless remote controls
Sony projection Glasses that allow you to play games in Vitual Reality and watch DVDs as well (or you can use the screen in the car)
Now the real Kicker is that I have installed and mounted everything without any drilling, cutting, or modifying on the interior. I can take everything out easily and I haven't even really compromised the trunk (still have the spare and sub removes in less than 10 seconds)!!!?#*???