I have always been a stock stereo kinda guy and thought they sounded fine. But I replaced my stock stereo with an eclipse avn2210p so I can have navigation and ipod connectivity. The sound quality was like night and day I was shocked at how restrictive the stock stereo was as far as quality of sound. If you are on the fence of changing your head unit do it. You can always go back if you don't like it but you will be amazed at the sound quality.
ETA: got it on ebay for $155 with the nav maps included.